There isn't a lot to go wrong with a turntable. It should be easy to find the problem. It has to be the cartridge, the cable or the preamp.
The cartridge would be the first thing I would check. You should find 4 small wires attached to the back of the cartridge and running into the arm. Check the connections there to ensure that they are not loose or corroded where the pins and connectors mate. Those wires are delicate and I've had one fail at the crimp after I manhandled it during a cartridge change.
If those look good, make sure the issue isn't in your phono preamp by swapping the phono output cables at the preamp inputs to see if the same channel is dead. If the same channel is dead that would indicate that your table is OK and the preamp is bad. If the dead channel moves with the connections to the preamp, your preamp is probably OK.
Swap the pairs on the back of the cartridge and see if the dead channel follows a particular pair of wires. If the dead channel moves, it's the wires through the tone arm. Use a DVM to check for continuity on each conductor of the dead channel. If you find no broken connection, check for a short.
I assume that you don't have a spare cartridge of you would have tried that. If nothing above isolates the fault, one of us can probably dig up a used spare for you to try. I think I have one but I have to dig for it.
If none of that helps, I don't know what else to try.