Windows NT evaluation problem

I have a serious problem regarding on Windows NT server.My original licences Windows NT server was installed in one of my computer but the problem is after 3 months it display out an message saying”Your Windows NT server is a evaluation version and had already expired and will be restart in 1 hour”.Can you please advice me on this case because I encountered this problem 6 months ago
and my server keep on rebooting after 1 hour of the notification error and I have to reinstalled my Windows NT server again to solve the problem and now the same problem is back again.My server is connected to network
Please advice.
Thank you.