It looks like Ralph may be right. Either it just
fell out or it snapped off inside. You should
be able to just loosen the cap (the one with
the ridges), push the top pipe back in, then
retighten the cap.

From the photo it looks like the pipe is not
actually PVC but rather the cheap, thinner,
plastic do-it-yourself stuff. If for some reason
you can't put the pipes back together as is, go
to Home Depot or Lowes, in the section where
you can find PVC, and you should be able to find
bags of the cheap plastic fittings for sale. They
come in a plastic bag with about 10 parts -- sort
of a kit for doing a sink drain. PVC and the thinner
plastic are not interchangeable.