Start by reversing the cable going to the amp- if the sound moves to the other channel, the problem is in the turntable and if it remains in that one channel, it's in the receiver.
If you have a multi-meter, unplug the cables from the receiver and measure the resistance from each terminal in the end of the tonearm to the ends of the cables. If you see high resistance or no connection, you'll need to find out why. Could be as simple as bad RCA plugs but some B&O turntables had a cable with two RCA plugs on one end and a round 5-pin DIN plug on the other. You can measure the resistance from the pins in the DIN plug to the ends of the RCA cable. DIN plugs are available online.
If the receiver is the cause, rotate (or press the switch for) the source selector several times while a record is playing- it may come back, but that would likely be short-lived. It means the source selector switch needs to be cleaned. Caig DeOxid is a very good product that's widely available at a reasonable price. If you do this, clean all of the controls and switches.
If it doesn't come back so easily, carefully remove the cartridge from the end of the tonearm and look at the four pins- if they look clean, re-insert it and try playing an album- if it comes back, it may just be a matter of the cartridge pins and their terminals being oxidized. You can clean these with the Caig, too- remove the carridge, wet a cotton swab with the liquid and dab it in the end of the tonearm so some of it can enter the terminals, then re-insert the cartridge.
