It could be a short on the circuit that powers the light but the fact that it came on when the fuel was low rather than coming on at a random time would make me think it's a problem to do with the sender in the tank. I'd try unbolting the tank and shake it around a bit to try and force the float in the tank to move. See if you can get the light to go off. If it won't go off try tracing the wiring that powers the light and look for a brake in the insulation or an area where it's rubbed through or melted together. Connectors can melt together internally too so separate any connectors in the circuit.
