It is a digital volume knob. There are no stops in either direction. After you pulled off the knob, which is purely cosmetic, was the post/control loose?
This might help you:
If it isn't the plastic control mount issue, this will give you an idea of how to replace the volume control, if they are available separately. This radio will be a little different in your car, but should be similar.
Do you get any sound at all? It may not be the volume knob.
Salty