The block explorer does still show a balance of 20,046.85666512 BLK.
Since the wallet has been not staking for a while (last one 2015/05/20), maybe you found a block once you went online again and the "missing" BLK are probably just not spendable at the moment? 

If that is not the case, you could try a rescan (just start the wallet with parameter -rescan for that).