The company said it was halting operations for 24 hours and was now investigating the incident.
"The incident has been reported to the relevant authorities and law enforcement and we are cooperating with them as a matter of urgency", it said.
Hackers have carried out a heist on a leading digital currency platform, making off with an unspecific amount of bitcoin.
While NiceHash has yet to place an amount on how much is stolen, a Bitcoin wallet address highlighted by NiceHash users that appeared Wednesday and has received a large deposit out of nowhere contains 4,736.42 BTC, valued at about $62 million at current prices. The company is now investigating the precise amount stolen, though estimates around the web put it north of $66 million United States dollars worth of the popular cryptocurrency (at time of writing 1 BTC = ~$13970.50).
Troubles with the website over the past day or so drew alarm and complaints, with many bitcoin owners posting panicked comments on NiceHash's social media accounts.More news: Small risk of breast cancer seen with hormone contraceptives
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"We understand that you will have a lot of questions, and we ask for patience and understanding while we investigate the causes and find the appropriate solutions for the future of the service", the company said in its statement.
Bitcoin has proved popular with currency traders, going on a massive bull run in 2017.
That compares with a value below $1,000 at the beginning of the year. Such currencies are not tied to a bank or government and allow users to spend money anonymously.
They are basically lines of computer code that are digitally signed each time they are traded.
Online security is a vital concern for cryptocurrency marketplaces and exchanges, with bitcoins contained within digital wallets that have increasingly become a target for hackers as the number of bitcoins stored and their value has skyrocketed over the past year. Nicehash needs the Bitcoin to repay users who offer up spare computing power for mining various coins.