I have a John Deere L110 and for some reason the drive belt has lost tension the idler spring is OK but the belt is just loose and the mower will not respond when depressing the foot peddles.

There is a belt tensioner bracket located on top of the deck. I would try to tighten the belt first, if you haven't already. If this doesn't fix the problem, you may be in need of a replacement belt. Here is a link to a short YouTube video (1:55) that will show you how to safely adjust the tension on your drive belt. Good Luck with it!

https://youtu.be/K5T-IDIZYuE

did you inadvertently release the transmission rod that sticks out the back? Just turn it and move it back in