Joggle the bulb gently in the socket. Those rubber sleeved sockets sometimes have a tendency to loosen up and not make good contact. If bulb is okay, as well as the socket, your next step is to use a test light to check for 12 volts. If there is no power, connect a black temporary test wire to the ground side of the socket, and the other end to a good ground. If it works, then you know it is a ground problem. If it still does not work, then you will need to check to see if it is getting 12 volts. Next step is to do the same with a red test wire to a good 12 volt source and to the positive side of the socket.