Most likely your fuel level sender is screwed up. This happened to me last winter but the cause of the failure was even more distressing. In my case somehow a breather line froze and the force of the fuel pump was strong enough to implode the fuel tank, when I looked at the bottom of the fuel tank it had a concave dent running almost the whole width of the tank. The collapsing of the tank was enough to ruin the fuel level sender inside the tank. I urge you to look at the bottom of your fuel tank to see if a similar thing has happened to you.