I've had this problem a few times. (edit : once ever after the most recent update)

I just ended up deleting everything except for wallet.dat in appdata/roaming a few times and after a few tries the wallet finished syncing with the network. A few times it just got stuck on a certain block and would never finish syncing.

Beyond that importing a backup wallet should fix it, but if you're willing to be patient it doesn't seem completely necessary.