I have the exact same. Broke on me last night. It will function as long as the charging cable is connected but I cannot turn it off then. As soon as the cable is disconnected, I cannot interact with it at all.
The headset is charging though according to synapse and was left to charge until completely full.

I wonder if its the batteries thats needs changing or it can be fixed, because its a real shame because I realy like them.

I am pretty sure it is a software related issue though. my synapse updated and all of a sudden my headset doesn’t work anymore without prior warning. 

I see, lets hope thats the problem and maybe I can go back to a older version and get them working again. If thats possible.