After a period of time a 'force stop' button will appear.
Do Force stop then restart.
If that fails then you will need to contact Nitrado Support.
Link: Support | nitrado.net
We have been doing the force stop to no avail since yesterday. We also created the Ticket, but there has been no answer. We already decided to change the server provider, but we still have a 1/3 of the time here left (2 of those days lost and I don't think we are going to get a refund).