need simple explaination for band theory of solids and conductor,insulator,semiconductor using band theory?

Because of translation symmetry, you have Bloch's theorem with arbitrary momentum within the Brillouin zone. This is what created bands.

On a single atom you'ld have no translation symmetry and therefore no generator for that symmetry (momentum is the generator of translation).

Then, the question of whether the material is conductor or not depends on whether eigenstates with energy around the Fermi energy exists (so that small perturbations are possible and electrons are easy to move).

If the material is not a conductor, the separation between isolator and semi-conductor is along a blurry line: isolators have their eigenstates "far" from the Fermi energy, while semi-conductors have eigenstates close enough so that it's still realistic to provide enough energy to move an electron from one state on one band to one state on the other side of the gap.

It's called "band" theory because we model electron energy levels as bands, strata, above a ground level. And the farther up the levels, the looser are those electrons bonded to the atoms that make up the material. At some level of energy, depending on the material, there are electrons at such an energy level that they are free to drift about, not bonded to the atoms. And these are the free electrons that will flow to make a current in a conductor.