i don't believe that is possible, did you cross the PO 'account payee'? who did you make it payable to? It's not the same as a cheque I don;t think cos the post office have your money and they honour the postal order, at a guess
You need to ring the helpline number (on your receipt).
It OUGHT to be possible (all POs are checked against a database at the PO before they are cashed) though I don't know if it can actually be done.
Under the old manual system you couldn't do it - you had to wait ? 6 months until the PO expired before they would check and give you your money back if it hadn't been cashed in the meantime.