Sign into Nitrado.
Go to "My Services".
Click on "Web Interface".
In the top left it will say "Server Started" or "Server Stopped". Simply hit the red "Stop Server" or green "Restart Server" to do whichever you need.
*note* You will HAVE to restart the server if you make any changes to your game settings. They won't be applied until a restart happens.
If your server says it's running and you can't find it, under "Settings" click "general" and look at your server name. Either search that or change it to whatever you want (which will require a server restart). And you should find.
Hope this helps.