How do I fix a disconected lever in my tub like the plunger lever came off

I am not sure what came off, but I will try to describe what the parts are and you can figure out how to repair it. This is based on the fact that I don't know what you have, I am just giving you a general view of how they work.
First you have a face plate with two screws holding it in place, In the center of the face plate you have a lever type switch that opens and closes the drain by moving a bucket up and down to close the drain, On the inside of the lever ( your can see this after you take the screws out) with a long "U' shaped piece of metal that is attached the arm with a cotter key, at the bottom of this "U" piece you have a threaded rod that goes through a hole in the "U" on this rod is a small nut used to lock the rod in place after you have the length adjusted to the right length to operate the drain correctly. At the bottom of this rod is a hook, in this hook is the hook of another rod and at the bottom of this rod you have the barrel that closes off the drain when the lever is in the up position. Most of the time the problem is that the lever arm breaks where the cotter key goes through the rod. You can buy a new face plate and it comes with the lever already installed you just have to place it back through the the "U" and replace the cotter key. If you need the other parts they can be purchased also

Are you asking about a lever that came loose or the stopper that's on the end of the linkage? Regards, Tom