You don't give much information to base a diagnosis on. Does it always read empty? If so make sure the connection to the tank sending unit is tight and the wire isn't broken somewhere between the sending unit and the guage. Does the needle move at all? Does it always read full? If so check that the wire from the sending unit isn't accidently grounded somewhere.
Broken or grounded wire to the tank not being the problem, then power the gauge by turning on the ignition and remove the wire from the sending unit - if the gauge reads empty and when you ground the wire from the sending unit it reads full, then the sending unit has failed. If the gauge doesn't move under either condition the dash unit has probably failed.
By a wide margin the problem is usually with the sending unit.