Akai Twin cassette player had sound but no sound now

Akai HX-M459W Twin cassette player had sound but no sound now.

Hi everybody, My AKAI HX-M459W separate was working other than needed to change belts ,did so and when I tried it the sound started to break up and then stopped all together so everything else working but no sound .have checked the wires and see nothing broken so where would I start to check what has gone wrong Thank you for any info

PS: should have said its a double Tape Cassette player / recorder, have the service diagrams for you to look at