We had a power surge today that restarted our Moodle server at Okoboji Community Schools. It just so happened that one of our teachers was regrading a quiz in Moodle when this surge and restart happened. This caused the mdl_user table in the MySQL database to become corrupted. To fix the issue I had to do the REPAIR TABLE mdl_user on that database. After that all seems fine.
I was panicked for about 5 minutes until I was sure the database was repaired properly.