@seanbarry most Sony cassette players fast forward and rewind run directly off a gear train. The Play function depends on a rubber belt to spin the heavy metal capstan (the metal pin that pulls the tape across the play head). If that main belt has stretched out, snapped, the play mechanism will not engage.
This is a general answer to a general question. We will need to know your cassette players exact model number to try and help you in more detail.
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