Q: How an electron can have different energy levels if it is once declared that the energy level is fixed? Let me be more clear about what I'm actually trying to ask. So, as we know for an isolated atom the energy level for an electron in $1^{st}$ shell is equal to $- 13.6$ $eV$ now if we shove that atom in a crystal now the energy level of that electron in $1^{st}$ shell is no longer equal to $-13.6$ $eV$. Now it will possess any amount of energy between the range of energy band (valance band for this case) because now it'll be under the influence of other charges, vacancies etc. which is specific for this crystal.
Now my actual question is, in an isolated atom if we give some amount of extra energy to the electron i.e more than $-13.6$ $eV$ then it will be excited. But if we shove that atom in the crystal then it can possess any amount of energy, so how is this possible? If that electron possesses more than $-13.6$ it should get excited but it doesn't and also it is under the influence of other atoms now. so it should possess energy more or less than $-13.6$ and hence should be excited or de-excited. In other words this happens due to the formation of energy bands but what makes this energy bands? someone might think that this is because of the influence of other atoms but this statement doesn't support the formation of energy bands completely. Just imagine a case in which there are no energy bands then as soon as we shove an atom, then that electron in $1^{st}$ shell will be excited due to the absence of energy bands. I'm not able to digest this point properly.

A: 
as we know for an isolated atom the energy level for an electron in ℎ shell is equal to −13.6 

This is not correct. -13.6 eV is the lowest energy level for a hydrogen atom. The second lowest is at -3.4 eV, and all of the rest (an infinite number, in principle) are between -3.4 eV and 0. The step size between energy levels is not uniform, and is not the same for different atoms.

what makes this energy bands?

As atoms become close together their electron clouds overlap. The atoms then form molecules and crystals and so forth. In these structures the electrons do not belong to individual atoms, but are shared in the molecular orbitals. So, for example, in the unexcited hydrogen atom the two electrons share the molecular ground state shell. By the Pauli exclusion principle they cannot occupy the same molecular orbital state, and with the two atoms the molecular potential becomes different with two "wells", so the -13.6 eV ground state is split into two slightly different energy levels. As more atoms share the orbital the state is split into more levels. In a crystal, where trillions of atoms share electrons freely, the energy splits are so tightly packed as to form essentially continuous bands.

A: You talk about the lower energy level for a H atom. For atoms with more nuclei, the electrons don't have the same energy level. If different atoms are far away as in a gas, electrons of different atoms have different states, just because they are independent.
In a crystal, for the electrons of the higher energy level of each atom, the influence of the neighboring nuclei can not be ignored. And different electrons can not share the same state, what would happen if they had the same momentum and energy. So they occupy the available states, starting from the lowest one, forming the band structure.
