Bathtub trip lever broken and detached
Remove the two screw on the trip lever cover, and have a look inside.  Since you have a plunger style, the linkage bars have likely dropped down out of view. You'll have to reach down the hole, and pull the assembly back up.


Straighten out a wire hanger; or some excess solid strand electrical wire, and put a small hook in one end.  
Go fishing.
Try to hook the plunger assembly, and pull it back up out of the hole.


For reference, this is about what the drain looks like.



There should be a ridge at the bottom of the overflow pipe, to prevent the stopper from falling down the drain. So the stopper and linkages should be somewhere in the overflow pipe.

I've heard of people using a shop-vac to stuck the plunger out, but I've never tried it myself. 

Good luck.