QC 35 II doesn't turn on
My QC 35 II are about 3 year old.
Only accidentally used when I was flying. Nowadays when flying is restricted I haven't used them for about 4 months. When I recently pulled my headphones out the battery was flat and I charged it to 100% and used them for several hours. Next day they didn't turn on. I put them on charge again and despite the amber led light is flashing it never gets to green (even after whole night of charging). I tried using different cables and different USB ports, including laptop and wall USB sockets, all methods lead to the same result. I used the cable and the method used by one of my work mates who's got the same model of headphones. It works for him, but doesn't work for me.
This means I can't turn them on. I can't reset them. I also can't update the firmware or follow any troubleshooting steps. PS Advertised ability to be used via cable on flat battery does not yield any result. Why is this?
Does this mean the battery is not holding the charge or possibly something else?
Is there a remedy or full replacement is the only option?