If you see outgoing transaction(s) sending the coins to a different wallet, then they've likely been stolen and your chances of recover are zero.
If you just don't see the correct balance, but there are no outgoing transactions, then it might just be a syncing/display issue.
As others have suggested, check your known addresses on a block explorer first. If the coins are still on your known addresses, then it's like just a syncing issue. If the coins have been moved, then it's more likely that your wallet was compromised and your coins have been stolen.