That is very strange... Presumably you stopped playing at some point during this time, so at least some of the saves must have worked before (in order for you to have been able to continue).
The game saves files to a temporary folder (using a .tmp extension) before moving them to a permanent folder. This is to prevent a working save from being damaged if there is a problem during the save process when overwriting an existing save.
Have you had any system crashes, done a system restore, or something, that Windows may have decided recent save folders were corrupt and 'fixed' them?
One person had a system crash 5 minutes after exiting the game, and during reboot Windows deleted the inv.b2 files from all their recent saves. Fortunately this is a zero byte file (perhaps why Windows decided they were corrupt) so it was easy to fix.
If you still have the corrupt saves, check the number of files in the saved folders (in '..\Divine Divinity\Run\savegames'). A new game should have 22 files, after going to either Verdistis or the Dark Forest it should have 27, after going to both it should have 32, after getting to the wastelands it should have 37, and after getting to the Black Ring dungeon it should have 42. Depending on the file, missing files might be replaceable with corresponding ones from the next most recent working save (or a new game), though this might cause other problems.
