From what I know, it's not possible to cancel an unconfirmed transaction. The closest thing would be a dojble-spend attack. Seeing as you sent it without a fee, it will most likely be returned to your wallet within a few hours/days. Just be patient and it'll come. On the other hand, it could confirm after a while but your only option right now is to wait and be patient. Always remember the fee, I know I've sent a few transactions without a fee and it took forever to get the Bitcoin transaction confirmed. Could you possibly post a txid so we could try to rebroadcast it or just check on the status?