ImportError: No module named 'Tkinter'

For some reason, I can't use the Tkinter (or tkinter, on Python 3) module. After running the following command in the python shell:

import Tkinter

or this, in Python 3:

import tkinter

I got this error

ModuleNotFoundError: No module named 'Tkinter'

or this:

ModuleNotFoundError: No module named 'tkinter'

What could be the reason for these errors and how can I solve it?