The moment anything is protected by a password, PIN code, or anything that needs to be remembered, there comes the time when your mind goes blank. Alternatively, the person who did know that password is no longer there to recall it when required. So how can you gain access to your Mac when you don’t know or can’t remember its password?
That rather depends on how it’s secured, which model it is, and whether its boot or data volume is encrypted.
Firmware password (Intel)
A firmware password is set and removed in Recovery, and can only normally be removed if you know the password. If your Mac still has removable memory, it’s possible to remove one stick and reset the NVRAM three times, but that’s not guaranteed. The most reliable way to achieve this is to take the Mac to an Apple store, together with proof of purchase or ownership, and ask them to remove the firmware password.
Read more at eclecticlight.co