My shower unit came of wall how can I fix it

What do you mean by 'shower unit.' The showerhead where the spray comes out? You buy some plumbing white thread tape at a hardware store and wrap it around the neck of the bent pipe coming out of the wall, then thread the showerhead back on and tighten a little bit with a wrench. You could try it without the tape to see if it leaks or not first.

Or do you mean the round control handle?

Need more info, please clearify.

What do you mean by shower unit, Thanks.

John