It sounds like you may have lost the ground to the tank, but it could also be the sending unit in the tank went bad. The guages are usualy durable, so they don't go bad as aften, but they do go bad. Also make sure all your fuses are in and good, even if it dos'nt pertane to the problem. Somtimes when a fuse is blown, the path to ground changes. Iv'e been a tech for 37 yrs. and seen this alot, but mosty on the newer cars.
Chuck