HP ProBook 6555b Notebook-PC - Manual (Page 87)

If the computer is unresponsive and you are unable to use the preceding shutdown procedures, try the
following emergency procedures in the sequence provided:
● Press ctrl+alt+delete , and then press the power button.
● Press and hold the power button for at least 5 seconds.
● Disconnect the computer from external power and remove the battery.
Shutting down the computer 77