Unlock iMac from black screen
My iMac 27” is locked with a black screen and gray lock icon and I can’t remember the password.
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Follow all of the steps at > https://support.apple.com/en-us/HT204455
If you forgot your firmware password
If you can't remember your firmware password, schedule an in-person service appointment with
an Apple Store or Apple Authorized Service Provider. Bring your Mac to the appointment, and
bring your original receipt or invoice as proof of purchase.