Isolate the wire coming from the fuel sender to the gauge. It should read +/- 40 ohms to ground when the tank is full and zero ohms when empty (in between will be 0 - 40 ohms). Does the gauge read "full" or "empty" all the time? Full would indicate that the sender is stuck and empty typically means that the sender is bad (open), the gauge is bad, or the wire between the sender and the gauge is bad.

The suggestion of a broken wire is probably a good one. You can take a length of #14 or #16 and bypass all the way to the sender unit and attach to the gauge and that will tell you if then wire is bad.