As Capt Cook mentioned, it might be full of water. That should be easy to check, and if you use a water indicating paste on a dipstick you will see how much water is in the tank. Kolor Kut is one brand of water indicating paste. The other thing to check is the vent. It might be clogged and this could make filling the tank difficult, especially if the fill nozzel is a snug fit in the fill pipe. Air can't escape.