restore your backup file if you can find one that is intact. Look in the folder for a recent larger file, the smaller file is probably an indication of the untouched world, but modification date may help too. Zip your entire folder before launching the game again too, in case it destroys what you have left (if anything). Turn the steam cloud off. It is a constant source of these kinds of problems.

keep manual backups of your characters and worlds out of their folder. Stuff happens.