When controlling a computer via ARD 2.0 or later that has lost power or has stopped responding, the control session may remain active on the Remote Desktop Admin computer.
You can dismiss the control session by closing the control window or clicking the "Exit control or observe session" button when using full screen mode. If the client comes back online while the control session is still active, it may automatically reconnect.
Note that the 'Trying to reconnect...' message does not display during this time.