Why do I have to replay the first level every time I open Dungeon Defenders?
Found a fix here, on the official forums.
Right click on the game in steam -> Properties -> Verify game cache When I did that it said "2 files failed to validate and will be reaquired"
Then I restarted my game and got all my progress back. I never had the problem with deleted characters or lost items though. I was just unable to access levels I had unlocked. It might be a steam updating problem.