Why has one socket stopped working?

Start with the simplest checks first. Check the fuse in the plug of the appliance you were using — a blown fuse is the most common cause. Check your fuse box for a tripped circuit breaker. Try the socket with a different known-working appliance to confirm the socket is the issue, not the appliance. If the socket is completely dead with any appliance, check adjacent sockets — if they all work, the fault is likely in the socket itself or its wiring. A loose wire connection behind the socket is very common, especially in sockets that have been knocked. Switch off the circuit at the fuse box, remove the socket faceplate, and check that all wires are firmly secured in their terminals. If you find a loose wire, tighten it — but if you're not confident doing this safely, call an electrician. Do not continue using a socket that shows scorch marks.