If it shows a reading after you fill it, I would guess voltage regulator.. If it stays on empty then I would check to make sure you have power back at the plug and at the gauge, and check your ground too.. If all that fails its either gauge or sending unit...
It's been discussed a number of times, I don't remember the details, so instead of guessing, I'm going to say, do a search and you should be able to find out how to check your sending unit and more.
http://mmb.maverick.to/showthread.php?t=69082&highlight=gas+guage&page=2 There is some good info on page 2 of this link for checking the guage. This is the type of thing DDB was talking about, he just has a way of saying things sometimes...
