You have lost ground to that socket. More then likely the socket is bad and will need to be replaced. But before doing that pull the socket out and you will see 3 wires. One for brake, one for running lites and one for ground that one should be black in color. wiggle that wire and if it starts working replace the socket if not using a jumper wire find a good ground connect one end of jumper to ground and the other end clip to a small flat screwdriver shaft then touch the screwdriver to the brass part of the bulb. If the lite works check the black wire for an open or follow the black wire back to where it is grounded to the frame and remove and clean.PS make sure you have the key on and the turn sig. handle on.