More than $70 million in bitcoin was stolen from NiceHash, a cryptocurrency mining company in Slovenia. NiceHash, which mines bitcoins on behalf of customers, said it is investigating the security breach. It will stop operating for 24 hours and encouraged users to change their online passwords. Andrej P. Škraba, head of marketing at NiceHash, said approximately 4,700 bitcoins were missing from the company’s wallet. He added that it “was a professional attack” but did not give any more details.