What happens after the migration to X11? Will the old coins be worth zero?
We are moving to another algo and a complete different blockchain. As such, coins on the old (current) Quark chain will not be functional any more. There will be a small transition period running the quark chain to aid moving coins over to the X11 branch, but don't take any chances, try to move them asap before we move this Sunday.
As for Yobit, I'm sorry for your loss. You can try to ask them to do something about their wallet software, but as BizzyB said, the dev team has asked them repeatedly for over a year to fix their software, but without any response. Your best option is to try to sell your coins, but then someone else will be holding a worthless asset once MUE changes to X11.