Why wont cassettes player won't play

@annecunningham we need to know what happens when you try. It will be helpful to know if you see the cassette actually turning (and if everything else on your "boombox" still works. If you do not see the cassette turning then my concern would be that this is a broken belt or a bad motor. Most likely is a belt. Here is the Sony CFS 204 and page 14 shows you where those belt are. Yes, it'll require a little bit of dismantling but it really is not to terrible.

Now, if you see the cassette moving but have no sound from those, then the issue is most likely with your tape heads. First thing you want to do, is to clean them with some cotton swabs and isopropyl alcohol. Use the guides from here Sony CFS-204 Radio Cassette-Recorder as well as the SM to get it fixed :-)