NiceHash has been embroiled in a massive Bitcoin heist over the last few weeks, where some $60 million+ was stolen, forcing the company to close its doors to miners, temporarily.
Well, the company has announced on its website that they are coming back online, where they said: "Dear Miners: Final checks are in progress and NiceHash.com platform will be back online and operational tomorrow, December 20, 2017 until 24.00 CET".
Once NiceHash is back online, you might need a guide to get you going again - it just so happens I wrote one in the days leading up to the hack.