More than likely you've lost the ground at your tank that the sender needs to complete the circuit. Does it have a short ground wire at the sender flange or was it getting it's ground through the mechanical connection of the tank to the body? You may have to add a ground wire. You can easily test to see if this is the problem by adding a short ground wire from the sender flange to a good chassis ground.

If all that looks good, check each connection and splice (if any) between the guage and the sender - something has become discombobulated!