We have an unresponding mediator on our dispute with a trade partner.
It was a revolut based transaction (me seller) where I was not online in the next 24h (I was not aware it was so short time) and now I can’t confirm the payout, we communicate with the buyer and both have no answer from mediators.
Who should I contact to get the mediator to answer ?
Don’t worry the arbitrator will pay him later from his own pocket. 
What’s the deadline until the arbitrator is punished ?
Not really. It looks like you will get punished unless you both talk with the mediator.
Use keybase/team/bisq. It’s a separate program install, but you will get a mediator within a couple of hours. I even did it on accident. It’s that easy.
Search: mediator or mediator3.