If your sender isn't grounded, the gas gauge generally will read full not empty. I would pull the sender out of the tank, ground it to bare metal and move the float up and down and watch the gas gauge. My guess is that the gauge mysteriously went bad, lost power, or the needle is hitting the gauge glass as mentioned above.
Carl

Last edited by 52Carl; 03/02/2014 5:50 AM.
1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission