If energy and mass cannot be destryed than what happends to the electrons of used electrical energy and why is this used energy unable to be converted back to electrical energy?

Answer 1

The electrons in an electrical current are not used up by the current. They are simply moved. A simple way to think of the current is as a stream of water, except that in the current it is electrons that are flowing, not water molecules. When the energy in a water current is tapped - by a turbine, say - the water molecules are not destroyed, although the turbine obtains energy.

In principle used ("converted" would be better) electrical energy could be changed back into electrical energy, except that some of it will be forever lost as heat. This energy has not been destroyed, merely converted into a form where it can do no useful work. This is an inevitable consequence of the second law of thermodynamics, which applies to all energy conversions, not just electrical ones.