The knob is not broken, but has been loosened internally.

 Solution

1. Remove the plug from the socket.
2. Remove the panel through which the button passes in the machine.
3. Put the knob through the opening, and turn the inner part (that is loose in the machine) with a quarter turn on the knob.
4. Lock the inner part by tightening the 2 screws that press against the metal sheet.
5. Refit the panel on the machine and the problem is solved.

This is caused by over-turning the button when unlocking. This causes it to loosen internally.