I am not much familiar with documenting/System designing stuff...

I have to maintain an application written in C# - working as windows service. However there is no documentation for this system which makes it really pathetic to find where some problem (conceptually) occured.

I would like to know the best way to design/document it (using current code) manually or preferrably automatically so that i can identify the exact problems...

like i feel as if sequence diagram won't much help probably.... Also, please guide me if i am taking it on wrong side?