You may wish to contact SE support directly about this, but...

1. Try a complete reinstall. Sounds like something really kicked out... so, I'd remove the whole game, and install again (And I don't recommend steam's repair feature for a game that has its own launcher/updater)
2. If the issue recurs... check for hardware issues. Mysterious corruption can be symptomatic of bad memory, but other things could cause it as well.

Those are all the tips I can offer from afar. Good luck!