Sometimes the game data just corrupts for no real reason, when it sits installed in the computer for a long time. This has happened to me on many games, many many times. Re-installing always helps in this situation, as it did in this case.
As Rather Dashing said, that error message is the most common type, and can mean about anything, so it's hard to determine the exact cause of the problem.
Glad the game is working again, though!