For some reason I've lost all data in my
.bitcoin directory except the
wallet.dat file. Now bitcoin-qt shows 0 balance, and it's 5 years behind sync. Is this possible to restore coins in this case?
If the blockchain files are gone, then your Bitcoin client has no way to match up the keys in your wallet with the transactions in the blockchain. Remember the balance is not stored in the wallet, but the keys in the wallet can be matched with transactions in the blockchain to calculate your balance.
Fortunately, the blockchain files are public and the same for everybody so you now just have to wait for them to download. It may take quite some time (days or more). You could try downloading a torrent of the blockchain to speed up the process, but this is optional.