There are two Windows utilities you can use to accomplish this.  The first utility is quesr.exe.  From a command prompt type in  quser /server:name of the server.    Once quser runs you will see all of the open remote sessions, and if they are active.  After you find the session you want to log off, you can then use the second utility.  This utility is logoff.exe.  From a command prompt type  logoff session name /server:name of the server.  Once the command runs, this will close the session, and now you should be able to log back in.