Determine who is logged in (and kick them)
mstsc /v:machinename /f /admin
or, if that doesn't work,
mstsc /v:machinename /f /console
Administrative Tools → Terminal Services Manager → Happy booting.
Log out as soon as you are finished booting. This is a special account that should only be used to boot off other users. If this account gets locked up, no one will ever be able to log on to the machine again. So log off and then log on normally to do whatever it was you wanted to do.