The volume shadow service decided to get rid of some old crap? Have a look at vssadmin from the command prompt. (Not sure whether that's something it'll do on its own...every once in a while, before I back up my VMs, I have vssadmin delete shadow copies, then followed by shrinking my dynamically expanding disks--very often, I'll find that an 80GB VHD drops down in size to 40GB)