Right ear in Sennheiser 461G is completely silent. Help?

One side not working is usually a case of a broken wire either as it goes into the jack plug or as it goes into the headphone, both places where it tends to get bent and stressed the most. If it's at the jack plug end you can chop the plug off with a couple of inches of wire and solder a new one on - but you need to get the connections right. If it's at the headphone end you'll need to find a way of disassembling it. You can get a cheap audio lead with a jack plug on each end, chop one plug off and use the remainder to replace the whole headphone lead.

As for disassembly, first of all, remove the foam ear pads if you can. You may then find some screws hidden under a soft covering, which you can locate by feel, then piece the cover with a screwdriver to undo them. It may still be held together wit clips. The iFixit jimmy is great for releasing these (or a similar tool, or even a knife, but take care") - insert it into any crack you can find and pry it open. But having got it open you may find you simply have to resolder a wire that has got yanked.