Why is my fuel gauge not working

Most likely the problem is with the fuel level sensor mounted inside the gas tank on the fuel pump. You can verify that by unplugging the fuel pump; with the car key turned on (but not started) you should see the fuel gauge go to empty.

If you take a piece of wire and short pins 1-2 you should then see your fuel gauge go to full. If that works as described, then you'll almost certainly need to replace the fuel sender. There are two ways to do this - first you can just replace the entire fuel pump; that'll be easier but quite a bit more expensive, or you can just replace the sender itself, but it'll take more work to disassemble the fuel pump and replace just that part.

Without knowing what year your car is I can only give you a possible example, but this is the sort of thing you'd be looking for if the fuel sender is the problem.

68145782AA - Genuine Mopar Parts