Hi and welcome to the forum. Are your other gauges working? Assume yes. If yes, this means the instrument voltage regulator is working and the problem is likely in thw connection to the sender, the wiring to the sender or the sender itself. What you can do is disconnect the wiring connector from the sender (Do you have an XR7? There are two pins in those connector/senders) and ground the connector to the body. If the gas gauge goes to full tank, then the problem lies with the sender. IF the gauge does nothing, the problem is with something forward of the sender, wiring, gauge, etc... (generally in that order).
Hope this helps.
Also, we like to know where people are from, put your location in your profile if you would and introduce yourself. Again, welcome, great forum and great members here.
Regards,
Bob