Most likely you will have to empty the tank, and remove the valve, and clean and inspect it.
Siphon the gas out of the tank (half a tank might be 7-10 gallons).
Many valves are all metal. If the metal is scored/pitted/etc, you might have to replace it. You might get away with simply cleaning and reassembly of the valve (if it only has removable dirt/deposits on the metal)
If the valve has a rubber/etc seal, that part might need replacing.
