Yeah I would do verify cache first especially if it was working fine for you in the past.
1: Open your steam library
2: Right click Dungeon Defenders (http://i.imgur.com/UyPCa.jpg)
3: Click properties (http://i.imgur.com/JvLz2.jpg)
4: Click local files (http://i.imgur.com/TO5wC.png)
5: Click verify integrity of game cache (http://i.imgur.com/IZDGD.png)
Also try switching to window mode/full screen and vice versa to see if it resolves it temporarily or not.
This worked. Thank you.