If you ground the signal wire that goes onto the sender unit the gauge should read max with the ignition on, if it does and you can get it out remove the sender unit and (a) check float has not dropped off or is full of fuel (b) check by earthing it and moving over the range and see if gauge moves. After that an Ohms meter and check the signal wire is not open circuit. If all the checks are OK all that's left is a faulty gauge I am afraid.
