To check for the failure cause, remove the left hand fender to gain access to the sender. With the key in the "run" position, unplug the wire from center of sender and touch it the stud that has the ground wire on it. If needle goes to full, sender is faulty. If nothing happens, using a jumper wire, jump the push-on sender wire to a ground on frame. If needle does not rise, plug wire back to sender and using your jumper wire go from frame ground to a stud on the sender.
If your gauge does not rise, remove the right hand fender to gain acccess to rear of gauge. You will need to remove a cover. Make sure the wiring is plugged tightly onto rear of gauge.
Failure is usually the sender or the wireing plug comes off rear of gauge.