Unfortunately in these cases for newer Mac systems shipped in 2011 or later, you have to get the system serviced to reset the password however, if you own a system from before 2011, then you can retrieve a forgotten password, provided you have admin access to the system. While this can be done using Apple's Firmware Password Utility on the recovery drive or OS X installation disc, there may be times when you do not have access to this drive either because it is missing or corrupted, and therefore cannot use the password utility. A firmware password on Mac systems locks the hardware to prevent booting to alternative modes that could bypass OS X security, but in order to access some of these modes you first have to disable the firmware password.
