Most gauges have indicators inside and outside.
The outside one on the tank is mechanical, relying on a magnet moving inside the tank. The needle should be showing 3/4 to full.
The inside one is electric, with a replaceable sender on the outside gauge.
First check the wiring to the tank sender. If connected, pop it out, or remove the 2 tiny screws, depending on brand, holding the sensor on the tank. ( no danger of leaks removing plastic sender )
Now you can use a magnet on the back of the sender to see if you can get the neddle to move. If it moves but doesn't change the tank reading inside it may be bad.
Connecting the 2 wires together will give you a full or empty reading. If that works, replace the sender with new.
Here is the snap in type.