There are 3 of the most common problems.
1. Defective Dash Voltage regulator (Voltage limiter) or gauge
2. Lost of a ground at the sending unit
3. Break in the wire going to the dash
2 Less Common problems
1. Actual bad Sending Unit
2. Fuel Gauge defective
The gauge and wiring can be easily tested using only a 12 volt test lamp, once you have access to the wiring at the fuel sending unit. This often requires draining the fuel tank and lowering it from the vehicle.
To test the gauge in the dash and it's related wiring, connect the 12 volt test light between the wire between the gauge and the sending unit (the one with power in it with the ignition on). Make the connection near the fuel tank sending unit.
Connect the alligator clip on the test lamp to a known GOOD ground. The bulb in the lamp should glow dimly. The dash gauge should read somewhere between 1/4 to 3/4 of a tank of gas. (Varies with make and model) This will verify that the gauge is either good or bad.