"ImportError: No module named Tkinter" even though I've used Tkinter programs as recent as yesterday and no substantial changes were made?
You may want to check if the version of the Python interpreter used to run the script got changed somehow.
import tkinter works in python 3 (instead of Tkinter).
This happens when you most likely have more than one Python installed on your system and you're running the program from a version different from the one you programmed it in.
By default, a Python program on your PC will usually be opened in the latest edition of Python.