Turn the unit back over and lift the top out of the tube that makes up the body of the unit.

There are two small plates (one plastic and one metal) behind the nut you removed. Mind the order of these. The metal plate is on the outside and the plastic is on the inside. Also note the orientation of the plastic plate. There is an inside face, and outside face.

Inside you will find the float and the three wires that make the sending unit work. You can see the three wires pictured here.

Inspect these wires carefully. If any of them is broken they will need to be repaired by soldering them back on to their contact points.

If these wires are intact, proceed to clean the internal components (wires, float, etc) and the inside of the tube with brake cleaner.

Be very careful when handling the insides of the sending unit as the three wires are very fragile!