The following video shows how to open and repair AKG K701 headphones. The procedure is the same for K702 and K601 as well as Q701.
If your AKG k701’s don’t work on one side you might find one of the cables have come loose.
You can easily fix this with a soldering iron. However if you don’t have one handy, you could try tape or glue. Just make sure the cables have contact and won’t come loose easily.
It will be obvious where the disconnected cable goes. And after you connect it your headphones should work just like new again.
Hope this helped.