Typically, you don't need to enter the BIOS in order to use an external monitor. It is a matter of pressing the function key for your system to toggle between the display options (LCD only, external display only, or both LCD/external).
If the links and suggestions privided by April_12 don't help, you will need to contact Acer directly for help. Be prepared to provide proof of ownership of the system in question.
On these forum's it is not permitted to provide advice on how to break/crack passwords.