You need to find out if tracking has shown delivery, if it hasn't you will have to refund your buyer.
Did you post using Royal Mail? Then, go on line to RM's Track and Trace. Enter the tracking number it will show if the item has been delivered or not. Forward this information to your buyer.
As said, if it hasn't , the buyer needs to open a case for item not received, and you fully refund through those case details.
Then, use your prroof of posting after the item has been missing for 10 days, to start a claim for compensation for yourself for a lost item, from Royal Mail.