Energy is defined as W = ΔE. Work changes energy.

 

Note that energy is defined as work is the change in energy. The value of the energy at any point is not defined; it is arbitrary. If we lift an object we do work, and the change of energy is the work. It does not matter if the energy is defined at the floor or at the table. Define it either way; just be consistent when doing the problem.

 

If the work is zero, then the change of energy is zero, or energy is constant. This is the law of conservation of energy.