How long ago did you send it? Have you asked the buyer to check with their local sorting office and neighbours as posties sometimes forget to leave a card. Items are kept for 18 days before being returned to sender.
You will need to refund the buyer sooner rather than later though because if the dispute is escalated and still showing as undelivered, it will be found in buyer's favour and it will be recorded on your seller dashboard.
RM don't consider anything lost until 15 working days after due date of delivery. Assuming you still have the proof of postage then you will be able to file a claim when time allows. You need a form P58 from the PO or can download from RM website: