[Bless]

Why are there no 2d and 3f orbitals?and why the 3s,3p and 3d orbitals have the same energy in the hydrogen atom but have different enegies in a manganese atom?

[Schrödinger]

In a hydrogen atom, the energies of the various orbitals depends only on the principal quantum number, n. Hence,the the with same 'n' are degenerate in a H atom.

[Schrödinger]

Azimuthal quantum number, 'l', can take only certain values. For every 'n', l varies from 0 to n-1.

orbital         l
  s             0
  p             1
  d             2
  f             3
  .             .
  .             .
  .             .

Hence in the case of n=2, the only possible values of 'l' are 0 and 1. Hence, only 2s and 2p can exist.
Similarly so with n=3. Only 3s, 3p and 3d can exist 

[fledarmus]

As I understand it, the differences in energy of the various orbitals in each shell has to do with the shielding of the nuclear charge by other electrons and interactions between electrons. Since a hydrogen only has one electron, there is no difference in shielding or interactions with other electrons. The degeneracy of orbitals is a special case for an atom with only one electron total.

This should also hold true with He⁺ (which also has one electron), but not with H^- (which has 2).