Did you include a payment ID, if the recipient asked you to? That is typically the only way merchants and exchanges can track transactions. Note that Monero transactions are permanent, and cannot be reversed, so your best bet is to contact the recipient, with the details of your transaction, and ask them to credit or refund you. Unfortunately there is nothing we can do to assist further, as the recipient already has the funds.
A random 256 bit hexadecimal string is important to include, it is simply random data that can help differentiate your transaction from other transactions that are sent to the same receiver.