One channel won't play

With it connected to the amp remove the wires to the cartridge. With the amp on touch each wire with your finger one at a time. On one wire you should hear a loud buzz on the left. On other wire the right. The two other might produce a minor buzz or nothing at all, so ignore them. If you hear the buzz on both channels there's something wrong with the cartridge or the connection to it.

If you don't hear the buzz. Then (1) there's a bad connection between the deck and amp (phono plug or wire). (2) failed pre-amp inside the record deck itself. (3) failed pre-amp in the main amp. (this would not affect any other settings in the amp so tape decks will play normal etc).