At this point I'm very surprised Yobit is still in business, as they've had so many complaints about them that you'd think people would have abandoned them by now--like I did.  They offer a lot of coins, but so many of them are dead, with no market, and their customer service is infamous for being shittier than a Port-O-Potty at a Van Halen concert. 

Don't know why they froze those wallets, but maybe it is for maintenance.  I don't know, and I'm sure that's all you're going to hear from them about it.  They just don't care about their customers, and they have got to be the shadiest exchange still in existence.  I've used them successfully in the past, but I'm done with them.  Haven't been on their site in many months.