Execute the following:
As you can see, you're seeing the logged in users, but you were busy and cannot login to the server or reboot it.
Alternatively you can execute:
qwinsta /server:<Server Naam>"
With the Session Name there's a Session ID attached on the same line.
With the following comand you can reset this session.
rwinsta /server:<Server Naam> <Session ID>"
Or you can execute the following command on the remote server (with PsExec for example)
This will stop all TS Sessions and work could get lost.
sc stop SessionEnv
sc stop UmRdpService
sc stop TermService
And to start these services once again:
sc start TermService
And you should be able to login to the server again.