I'm guessing your setup is much the same as the one in the Quicksilver 25 litre portable fuel tanks with a float on a wire arm which moves a piece at the top with a line on it to show the level.
Have you taken it out of the tank yet to have a look at it? - possible the mechanism has jammed, the float has come off or the float has developed a leak, filled with fuel & sunk.
These sort of things:
https://www.ebay.co.uk/itm/233848766195
and
https://www.ebay.co.uk/itm/274945125024
Albeit the second one has a plastic arm.