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Well, 4 years ago, when I bought my guitar, there was a plastic over the pickguard.
5 minutes ago I realized that I've never got out the plastic completely, and there still was some left under the knobs.
I pulled out all knobs without problem except one knob. That knob came out with the metal piece that holds it and I can't get it back.
Here's a photo: 
Yes, I'm afraid that's broken. It's not just a "knob holder", the body of the potentiometer (variable resistor) sits underneath the pick-guard, with just one part of it sticking up for the knob to attach to. Out of the guitar, it should look like this:
[Image by Iain Fergusson, licensed under CC BY 2.5]
Inside the squat cylindrical part is a rotating metal wiper - turning the knob moves this wiper along a resistive "track", varying the resistance between the middle connection tab and the outer two. This is the important part, the knob on top is just "window dressing" and not actually necessary for the electronics to work.
If the top part has come out entirely, as yours has, it's likely not possible to reattach it. The good news is, potentiometers are reasonably inexpensive (for a Strat-type tone control, you probably want a 250kΩ logarithmic/"audio" taper potentiometer, but if there are markings on the other pots try to get a match with those) and the circuit is fairly simple (although you might want to find someone to help if you're not comfortable with a soldering iron).
