One of 2 things is wrong. Obviously the gauge itself could be bad - although it's rare. And more commonly although it sounds strange is that your sending units are shot. The sending unit is an electronic device that is attached to your fuel pump - it shoots a signal to the gauge telling it how to read. Your fuel pump is mounted on the top of your fuel tank and hangs down inside of it. You may only be able to buy a fuel pump and sender assembly - it's not even worth the labor of replaceing one without the other anyway as the fuel tanks need to be dropped. Consider the age of the vehicle and the possibility of rotted fuel tank straps as well. I know it sounds strange that both be shot - but I've been burned too many times to count by saying, "Oh, it's gotta be the gauge if both aren't working" The other possibility is a damaged wire - but this would be the most uncommon. Good Luck!
"Life's too short to drive slow!"
My first name is Anne - was a Ford Dealer Service Manager for 11 years. Waiting for my new Expedition to come in (woo hoo!)