Getting "Your Trial Has Expired" despite owning the game on disk

This just started today. I got the game for Christmas, but had the demo before it. When I go to play now, I get the "Your trial for this game has expired" message. I checked the disk, and there's not a scratch on it. My other disks for games work. This is my son's favorite game and he plays it with all of his friends and he's in a bit of a panic now. Tried various things on the web but nothing's worked.

Please help!