Clean each side, generously silicone it back in place, and wait a day to use it.

I wager the handle is 2 part and held together by a snap ring. Once snap ring is pried off the piece with square hole pulls out of handle and you will find a screw lying loose inside.

Once it's apart it will be obivous how piece with square hole can be attached to square stem in wall with screw. Put snap ring back in handle then slide handle on until ring seats.

I would really,,,really apreciate you letting me know if this was correct.

Thank you both. It appears there is supposed to be a screw, but I can’t find it anywhere. I will head over to Home Depot to see if they have it there.

Or find a local real plumbing supply, they will have it no issues.

I'm confused as to why you are trying to buy a screw unless you can see how to install it.. Am I to believe you separated two pieces and there wasn't a screw lying loose inside?

Is it a friction fit onto that white piece, which looks like it's worn down a bit?

Does the handle separate from the bell-shaped piece? It looks like two different pieces.

Really, if you provided a manufacturer it would be a lot easier to give advice. At the very least, give us the number that's stamped inside the handle.

Thank you all. A plumber informed me it is likely a Delta shower - which I couldn’t remember - researching it looks like there is such a thing called a “set screw” - it does look like there should be a screw holding both sides together but I can’t find it, so maybe fell and went down the drain. I will be taking the piece to Home Depot - where it was purchased many years ago, and take a look at the Delta shower handles and screws. Thank you all for your comments.

If there's a set screw, then there's a hole for it. You said it doesn't look like there's anything that can screw on. So, is there a hole or is there not? You never answered the question of whether the bell-shaped portion separates from the handle.