Rossi, what you probably had happen is a wire going to the level gauge is faulty. Is the level gauge failed to "E" or to "F"? If on "E", then you have shorted wiring somewhere. If on "F", then you have an open. This is atleast a starting point for you.

Now, what you will need to do is to access the top of the fuel tank again (I'm sure not the words you wanted to hear, but you probably knew they were coming). Once you have access to the top, you want to find two wires, a black wire and a white/black wire. These two wires are what feed your fuel level sensor. What you will need to do is unplug these wires from the fuel tank and then connect a variable resistor between these two wires (this can also be done with a few set value resistors too). First set the resistor to 160 ohms. With the ignition switch in the RUN position (motor not running), the gas gauge should read full. If you lower the resistor down to 20 ohms, it should read empty. If you set the resistor to 90 ohms, should read about half. If you get all 3 of these readings, then you have confirmed a bad level sensor in the tank, replace the sensor. If you are not getting these readings, then it is a wiring issue between the computer and the tank more than likely. If this is the case, let me know and I will give you more things to look at and we will get you back on the road.