Question

prove that energy cannot be created nor be destroyed

Answer 1

Because the energy remains constant

Answer 2

The conservation of mass states that energy can neither be created nor be destroyed.

Energy can be transformed from one form to another.

For example, if we take some hot water in a beaker and leave it for some time, it's temperature will decrease, it will have the room temperature.

In the above example heat is released and water gets cooled down.

But anyhow can we collect the heat which was released to heat the water back???

No  

The energy from the convenient for gets transforms into less convenient.

So we hear energy crisis.