After prying off the cap, and loosening the collet (nut) visible inside the knob, then you should be able to lift the knob up off of the shaft.
From what you've said, it sounds like you have not actually removed the knob. If the knob is still on, and that's what spins all the way around, then you probably haven't hurt anything. Try loosening the collet a little more, and see if you can get the knob off, turning and lifting at the same time.
Once the knob is off, you should see the shaft sticking up out of the instrument, with a nut on it. This nut is what keeps the pot itself from spinning, and is probably loose. You'll need to try to figure out where the center position of the pot should be, then tighten this nut. It may help to remove the back plate so that you can hold the pot while tightening the nut, but just get it snug, don't overdo it.