Did you close CMD window on Server core?

Yes, in some cases this can be a problem. As is true, that if you are connected in remote to a computer, you can just press CTRL + ALT + END and start a new window in Task Manager, this becomes more difficult if you are remote connected in remote session. In This case this shortcut will not work.
Well, the problem is pretty easy to solve:
Run PowerShell as Administrator on some other computer and find the session ID with command:
QUSER /Server:ServerName
You will see all active connections and here you will find also a SessionID for the connection that you want to kill.
Now you know your SessionID and finaly you can kill the connection. This can be done in two ways:
– As command in CMD: logoff Your_SessionID /server:ServerName
– In PowerShell: Invoke-RDUserLogoff -HostServer ServerName -UnifiedSessionId SessionID
Well, now you lose your session and if you will connect to that server, you will have a new session with your CMD window opened.