The left side of the headphone stopped working

I've had this problem too.. The issue I had seemed to be related to the two contacts of the speaker in the left ear shell. There are two springs that press on two contacts on the pcb. They seem just barely long enough so even the slightest play in the plastic would cut the sound off. I soldered two small blobs on the contacts so they would make better contact with the springs and that solved the issue for me. To open up the shell there are 4 clips under the cushion at a 45 degree angle to the stem. Press two of these in with a small screwdriver from the outside to open it up. After that remove the 3 screws and then you can remove the speaker part and see the contacts.