It has been observed on Sun Fire headless systems (such as the 280R and the V240), that when the console is left disconnected for an extended period of time, the system can stop sending output to the port where the console monitor would be connected. Then, when a monitor is reconnected to the port, console messages are not sent to the monitor.
If this happens, you may be able to log in to the system remotely to effect a graceful shutdown and restart of the system. Console messages are then redirected to the monitor when the system comes back up.
If, however, you are not able to log in remotely, you must power down the system manually by pressing the power switch/button.
Because this problem involves a complete power down and restart of the system, you should plan to do this during non-peak hours, if at all possible.