I can't be specific which event might have caused this to happen. Too much could have happened between me noticing the problem and the first time the problem could have occurred.

Here's what I'ved checked so far but got me no results in bringing back my main menu


 - logged in as the administrator and made sure all of the permissions where switched on.
 - rolled back to the database and code folder from appr. 24 hours ago.
 - See if block called 'main menu' is still in the appropriate region under admin->structure-blocks. This is the case.
 - Placed the block 'main menu' in a different region to see if the html would show there. Nothing showed. 
 - Moving a block different than 'main menu' (in this case the stock 'navigation menu') into the region where I want 'main menu' to show. Navigation menu came up fine. This confirms that nothing is wrong with the region in question.
 - Went and see under admin->structure->blocks where I clicked 'configure' of the 'main menu' block. All visibility settings were normal and shouldn't be causing dissapearing. 
 - Compared the page.tpl.php file to other page.tpl.php of websites that does show the horizontal nav menu and everything looked fine.
 - installed menu_block module. Nothing changed.
 - Made a menu from scratch, which worked. Until after a few refreshes menu items started disappearing again. And yes, cleared caches, disabled caching, checked visbility settings checked permissions again and again.
 - Checked if the blocks were set to correct theme. 
 - Discovered query logging in the `devel` module. Not that I very well know how to interpret it but what I can share maybe are the queries that take longer than those standar 5ms


----------
**Here are the query logs that take more than 5ms**

 - `11.72 5 DrupalDatabaseCache::getMultipleP A E
SELECT cid, data, created, expire, serialized FROM cache_bootstrap WHERE cid IN (:cids_0)
default`
 - `9.19 1 language_listP A E
SELECT * FROM languages ORDER BY weight ASC, name ASC
default`

 - `33.16 5DrupalDatabaseCache::getMultipleP A E
SELECT cid, data, created, expire, serialized FROM cache_bootstrap WHERE cid IN (:cids_0)
`
 - `11.552 5DrupalDatabaseCache::getMultipleP A E
SELECT cid, data, created, expire, serialized FROM cache_menu WHERE cid IN (:cids_0)
default`

 - `16.17 5DrupalDatabaseCache::getMultipleP A E
SELECT cid, data, created, expire, serialized FROM cache WHERE cid IN (:cids_0)
default`

 - `21.08 2DrupalDefaultEntityController::loadP A E
SELECT revision.vid AS vid, base.uid AS uid, revision.title AS title, revision.log AS log, revision.status AS status, revision.comment AS comment, revision.promote AS promote, revision.sticky AS sticky, base.nid AS nid, base.type AS type, base.language AS language, base.created AS created, base.changed AS changed, base.tnid AS tnid, base.translate AS translate, revision.timestamp AS revision_timestamp, revision.uid AS revision_uid FROM node base INNER JOIN node_revision revision ON revision.vid = base.vid WHERE (base.nid IN (:db_condition_placeholder_0))
default`

 - `18.67 5DrupalDatabaseCache::getMultipleP A E
SELECT cid, data, created, expire, serialized FROM cache WHERE cid IN (:cids_0)`

 - `8.11 2metatag_metatags_load_multipleP A E
SELECT 1 FROM metatag LIMIT 0, 1`

 - `21.72 1_system_date_format_types_buildP A E

SELECT dft.type, dft.title, dft.locked FROM date_format_type dft ORDER BY dft.title
default`

 - `18.2 1system_date_format_localeP A E
SELECT format, type, language FROM date_format_locale
default
`
 - `
18.11 1i18n_menu_initP A E
SELECT ml.*, m.*, ml.weight AS link_weight FROM menu_links ml LEFT OUTER JOIN menu_router m ON m.path = ml.router_path WHERE (ml.menu_name IN (:db_condition_placeholder_0, :db_condition_placeholder_1, :db_condition_placeholder_2, :db_condition_placeholder_3, :db_condition_placeholder_4, :db_condition_placeholder_5, :db_condition_placeholder_6, :db_condition_placeholder_7, :db_condition_placeholder_8, :db_condition_placeholder_9)) AND (ml.link_path IN (:db_condition_placeholder_10, :db_condition_placeholder_11)) AND (ml.language IN (:db_condition_placeholder_12, :db_condition_placeholder_13))
default`
 - `20.36 1DrupalDatabaseCache::getMultipleP A E
SELECT cid, data, created, expire, serialized FROM cache_admin_menu WHERE cid IN (:cids_0)`
 - `25 1DrupalDefaultEntityController::loadP A E
SELECT base.fid AS fid, base.uid AS uid, base.filename AS filename, base.uri AS uri, base.filemime AS filemime, base.filesize AS filesize, base.status AS status, base.timestamp AS timestamp, base.type AS type FROM file_managed base WHERE (base.fid IN (:db_condition_placeholder_0, :db_condition_placeholder_1, :db_condition_placeholder_2, :db_condition_placeholder_3, :db_condition_placeholder_4, :db_condition_placeholder_5, :db_condition_placeholder_6, :db_condition_placeholder_7))`
 - `42.8 1PagerDefault::executeP A E
SELECT COUNT(*) AS expression FROM comment c WHERE (c.nid = :db_condition_placeholder_0)
default``
 - `6.79 1PagerDefault::executeP A E
SELECT c.cid AS cid, SUBSTRING(c.thread, 1, (LENGTH(c.thread) - 1)) AS torder FROM comment c WHERE (c.nid =`
 - `16.25 1node_tag_newP A
SELECT 1 AS expression FROM history history WHERE ( (uid = :db_condition_placeholder_0) AND (nid = :db_condition_placeholder_1) ) FOR UPDATE
default`
 - `8.681node_tag_newP A
UPDATE history SET timestamp=:db_update_placeholder_0 WHERE ( (uid = :db_condition_placeholder_0) AND (nid = :db_condition_placeholder_1) )
default`
 - `19.761_block_load_blocksP A E
SELECT b.* FROM block b WHERE (b.theme = :db_condition_placeholder_0) AND (b.status = :db_condition_placeholder_1) ORDER BY b.region ASC, b.weight ASC, b.module ASC
default
`
 - `8.021block_block_list_alterP A E
SELECT module, delta, rid FROM block_role
default``
 - `4.671node_block_list_alterP A E
SELECT module, delta, type FROM block_node_type
default`
 - `35.741i18n_block_block_list_alterP A E
SELECT module, delta, language FROM i18n_block_language
default``
 - `6.2425DrupalDatabaseCache::getMultipleP A E
SELECT cid, data, created, expire, serialized FROM cache_menu WHERE cid IN (:cids_0)`
 - `5.9125DrupalDatabaseCache::getMultipleP A E
SELECT cid, data, created, expire, serialized FROM cache_menu WHERE cid IN (:cids_0)
default`
 - `27.471menu_tree_check_accessP A E
SELECT n.nid AS nid FROM node n WHERE (n.status = :db_condition_placeholder_0) AND (n.nid IN (:db_condition_placeholder_1, :db_condition_placeholder_2, :db_condition_placeholder_3, :db_condition_placeholder_4, :db_condition_placeholder_5, :db_condition_placeholder_6, :db_condition_placeholder_7, :db_condition_placeholder_8, :db_condition_placeholder_9, :db_condition_placeholder_10)) AND (n.language IN (:db_condition_placeholder_11, :db_condition_placeholder_12))
default`
 - `17.2925DrupalDatabaseCache::getMultipleP A E
SELECT cid, data, created, expire, serialized FROM cache_menu WHERE cid IN (:cids_0)
default`
 - `15.441devel_node_access_block_viewP A E
SELECT na.* FROM node_access na WHERE (na.nid IN (:db_condition_placeholder_0, :db_condition_placeholder_1)) ORDER BY na.nid ASC, na.realm ASC, na.gid ASC
default`
 - `12.542DrupalDatabaseCache::getMultipleP A E
SELECT cid, data, created, expire, serialized FROM cache_metatag WHERE cid IN (:cids_0)
default`
 - `5.862DrupalDatabaseCache::getMultipleP A E
SELECT cid, data, created, expire, serialized FROM cache_metatag WHERE cid IN (:cids_0)
default`
 - `6.815DrupalDatabaseCache::getMultipleP A E
SELECT cid, data, created, expire, serialized FROM cache WHERE cid IN (:cids_0)
default`
 - `8.711DrupalDatabaseCache::getMultipleP A E
SELECT cid, data, created, expire, serialized FROM cache_variable WHERE cid IN (:cids_0)
default`
 - `19.161DrupalDatabaseCache::getMultipleP A E
SELECT cid, data, created, expire, serialized FROM cache_token WHERE cid IN (:cids_0)`
 - `11.871menu_local_tasksP A E
SELECT menu_router.* FROM menu_router menu_router WHERE (tab_root = :db_condition_placeholder_0) AND (context <> :db_condition_placeholder_1) ORDER BY weight ASC, title ASC
default`
 - `21.811_node_revision_accessP A E
SELECT COUNT(vid) FROM node_revision WHERE nid = :nid`
 - `8.5225DrupalDatabaseCache::getMultipleP A E
SELECT cid, data, created, expire, serialized FROM cache_menu WHERE cid IN (:cids_0)`
 - `5.891commerce_product_ui_commerce_product_type_infoP A E
SELECT * FROM commerce_product_type`