The fee included with your transaction was insufficient. If you divide the transaction fee by the size of the transaction in bytes, you will find that it has a fee rate of only ~3 satoshis/byte. According to 21.co a fee rate of 110 satoshis/byte is currently necessary for a transaction to be included in the next block.

The good news is that your money cannot be lost. If the transaction is not confirmed the money will eventually revert to the sender (you).

Note that blockchain.info is notoriously bad at estimating the required fee, which is why it says your transaction has a medium priority, although in reality it has only a slim chance of being confirmed.