Does your heater blower motor work? If so, you are getting power to the fuel gauge. If not, you may not be getting power to the gauge. A green wire goes from the 35-amp fuse to the gauge, and from the gauge to the blower motor switch.
As mentioned by Roscoe, a bad ground at the sender is also common, especially if the fuel line from the tank to the fuel pump has been modified - run a ground wire as suggested.