Left wallet qt alone for some months, then started it up and during sync coins disappeared?
There is a chance that someone took your coins by hacking into your computer and copying your wallet. However, if this is the case, you should be able to load your wallet and see the out payment transactions.
Alternatively, search your payment address at BlockChain.Info BlockChain.Info is possibly the best block explorer for bitcoin. It's a good place to do research.