> Importantly, our payment system was compromised and the contents of the NiceHash Bitcoin wallet have been stolen. We are working to verify the precise number of BTC taken.

That should be easy to find via the transactions. Are they still in your wallet? What's the address? If they are still in there, then use a backup key to move the BTC now. Do you have a backup of the keys?

Being that it is connected to a payment system, it's surely the hot-wallet. No mention of a cold-wallet makes it seem they've been completely wiped.

Multi-edit: Stream of consciousness