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.