Well that sucks your balance disappeared but dont panic, there are many fixes you can try and see if that work for you,

-Backup wallet
-download latest wallet and let it sync fully and replace with your wallet and see if the balance comes back or not
-try running wallet with -salvagewallet command
-try opening wallet with -reindex
-try repairwallet command and repair your wallet
-dump privkey and import privkey to new wallet

I am sure at least one fix mentioned above should fix your problem, so give it a try