Toilet is running when I flush but not flushing. How do you fix that?

It is an apartment toilet not a home toilet. It is attached to the wall, it doesn't have a back. It doesn't have a back part it is connected to the wall. Its not constantly running, it is just not flushing when I push down on the flusher. The water will run when I flush, but it doesn't flush. It just runs until I let go of the flusher.