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Is this the command you tried?
Shut down your Mac.Locate the following keys on the keyboard: Command (⌘), Option, P, and R.Turn on your Mac.Press and hold the Command-Option-P-R keys immediately after you hear the startup sound.Hold these keys until the computer restarts and you hear the startup sound for a second time.Release the keys.
Also, if you hold down the left shift key on your mac when you press power, and keep holding the left shift key.. Will it boot into safe mode?
Dustin S.Senior Technician and Network Switching Systems Operator / Maintainer.