Where can i find a key to the sewer? Nagur wont give me his quest cause i joined the militia and I read that his quest leads to another that involves the sewer key. Any way to get the key? Or am I out of luck?

If you can't even get Nagur's quest then you're pretty much out of luck, as to get the key you need to complete Nagur's quest plus two others (either working for or against the thieves guild in each quest).

Strange thing happened, i was fighting with Alrik and while sidestepping his attacks, I fell into the floor and into the sewer.

I killed the guys in the sewer and got what i wanted, the strength amulet. Glitches are fun!