Yep, if you reformatted or installed a new hard drive, you'll need to reactivate the games. I don't own any Bigfish games, but I have downloaded games from several other sites. Most of the time the same key can be reused up to a limit set by the publisher.
I ran into a similar issue when I upgraded to Windows 8. Apparently, even a new OS invalidates the activation. I had to activate them again when I bowed out of the Windows 8 thing.
I have all my digital downloads in their own folders (including the activation keys) and back them up to an external drive periodically. Reinstalling them again is a snap when all of the required files are in one place.