THINGS TO CHECK IF YOUR TURNTABLE HAS A DEAD CHANNEL
1) The amplifier's phono stage may have a dead channel, or the input
connector may not be making proper contact. Try switching left and right and see
if the problem then becomes the left channel. That will prove weather the
problem amplifier or turntable.
2) The output cable is not completely plugged into either the amplifier or the turntable. Try wiggling the connection whilst playing. See if sound either comes back or is intermittent in the dead channel.
3) Try another cable, but keep the ground wire connected between amp and turntable. If you don't, you may get lot's of hum.
4) Take off the headshell and examine the four little wires from headshell rear to cartridge. Are they all connected? Give each a little wiggle anyway, as the crimped on connections may be intermittent.
5) The headshell contacts may be dirty. Take off the headshell and rub the 4 little connection points on a piece of cloth for about 10 secontd to burnish them.
6) Try a spare known good headshell/cartridge assembly. You may have to borrow one from a friend. This will prove if the cartridge and/or headshell has a problem
If all this fails, then the problem can be deeper than we thought, i.e., internal. There are four wires + 1 black ground wire that extend from the arm's headshell connector all the way to the output connector. A wire may have become disconnected for some reason.