Quick tests:
1. short out the wire that goes to the sender to any convenient ground. The gauge will then go (slowly) all the way over to 'F' if it and its wiring are OK. If not, check the continuity of the wire from the sender up to the gauge, and of course check all fuses to be sure power is getting to the gauge.
If that test passes, the sender is dead.
2. disconnect the sender from the loom at the plug, connect a multimeter set to 'ohms *1' range between the sender wire and ground. It should show a fairly low resistance in the order of tens of ohms, which varies down to nearly zero for a full tank. You may want to pull the sender out so you can operate the float by hand to check that the resistance varies smoothly with angle.
That should help you localize the problem.
