DIY amateur here. The single handled plastic knob that controls the bathtub faucet recently fell off, along with a screw from its interior and a cap covering the screw. I wish I could post a picture, but the site isn't letting me upload them yet -- even though I have plenty of reputation on the other stack exchange sites! (UPDATE: I now have the reputation to post pictures! See below).
A metal square receptacle remains in the center of the knob-less handle (I guess where a long screw affixed the knob), and the faucet itself still works fine; temporarily, I've put the old broken knob on the fixture and control the water flow. Of course, it keeps falling out.
I'd like to know if there's a simple fix I can do, or do I need to replace the entire fixture. If the latter, how difficult is this to do? I don't want to have to break tile, etc.