Did nicehash gave you a TXID of the withdrawal? Can you post it here?
If nicehash didn't, it is entirely possible that your withdrawal is still pending and they still haven't sent it. That also explains why the cancel withdrawal button is still there.
The email where they stated it was successful was probably because they just quequed your withdrawal order. Anyways, It will be better to contact their support regarding this.
How long does it take to process withdraw request?
After you have successfully confirmed withdraw request, it may take up to several hours for withdraw to be executed (shown on blockchain). After that, it may take additional few hours for blockchain transaction to be confirmed.
EDIT: If its just an hour ago. Better wait as they have said it will take several hours for withdrawals to be executed. Contact their support if at least 24 hours has passed