First, give it some time, perhaps the sender arm inside the tank hung up somehow and will free up with use of the car. If not, then...
The short check is as follows:
Disconnect the sending unit feed wire at the tank sending unit (should be a tan wire), turn the ignition key on and wait a few moments (some gauges take awhile to react)... the gauge should read past full. Touch that wire to frame ground and the gauge should read empty.... as an aside, if the tan wire grounds somewhere in it's length, this will be the symptom.
If the gauge does NOT read as stated above, you have a bad gauge or a wiring problem between the gauge and the tank sending unit, or a bad ground.
If the gauge does read as stated above, you need to check for a bad sending unit at the tank. Carefully note where the wires connect so you can put them back in their proper position later (it's possible to trash the gauge if you don't). Turn the ignition off when connecting or disconnecting wires. Disconnect all 3 wires from the sender at the tank, and look for corrosion that would interfere with a good connection.
Then use the ohm scale of a multimeter to check the impedance between the tan wire's connection terminal on the sending unit and the other non-ground terminal on the sender (the terminal for a green or pink wire... the ground wire is black). The sender is basically a variable resistor, so the resistance through it varies by how much gas the tank contains; the reading should be somewhere close to 90 ohms when the tank is full. If you wish to check further, use a stiff non-ferrous wire (ie, copper, to prevent the possibility of sparks) with a hook on the end and have someone else use the wire (through the tank filler hole) and a flashlight to manipulate the sending unit float arm inside the tank through it's full travel... the multimeter reading should vary smoothly from near 0 ohms (empty) to 90 ohms (full).
Also while the wires are still disconnected, use the multimeter to check the connector on the black wire to a good frame ground to make certain that wire is actually connected to ground.
