Sennheiser HD280's left channel produces no more sound *need help*

You probably have a short. How long have you had them? If it's a long time, routine wear and tear. If hardly any time at all, a possible defect (cold solder joint, kink in wire, whatever).

If new(ish), RMA. If not or if you just want to DIY repair for the fun of it, the first step is probably to find the fault via multimeter. If you can confirm that the defect is in the cable and not the driver, a standard recable job will fix it.

RMA = return manufacturer authorization = return or exchange it. Generally used to refer to returning things to their manufacturer rather than to a retail outlet.

Multimeter = a meter with multiple functions. Implicitly an electrical digital multimeter which has functions like voltage measurement, resistance measurement, current measurement, and continuity testing. An essential tool for finding faults in electrical circuits.

If it's been in storage a long time, an RMA is probably out of the question - although some OEMs do have extraordinarily long or even lifetime warranties. (OEM = original equipment manufacturer = the people who made the thing rather than the people who sold it or rebranded it. If stuff breaks, you RMA it to the OEM.) Koss comes to mind - pretty sure theirs is lifetime on at least some products. I don't know about Senny though. I doubt it, but it can't hurt to check their website.

You can get a usable multimeter starting at about $5 (and going up to thousands). Basic electrical troubleshooting is one of those fundamental skills which everyone should try to pick up anyway. Modern civilization is founded on this stuff, but hardly anyone bothers to learn it anymore, any more than most people can maintain and repair their own car beyond filling the tank and checking the oil.