One order not arriving will not have a negative effect on your account as long as you handle it properly.
First of all, if you haven't already, check the tracking on the Royal Mail website.
Then, ask the buyer to open an 'item not received' case.
Edit
It looks as if you posted with Simple Delivery (label provided by Ebay although you paid?) so, in the event of such a claim, Ebay will deal with it so you won't need to refund the buyer.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)