So you need to make your Mac boot a different bootoption – But you can remember the cryptic key combination – Don’t worry, you’ll get them here:
Hold down Command and R during boot
Hold down alt and option key
Hold Command, Option, P, and R keys after hearing the startup chime.
Hold the Command and V keys simultaneously
Single User Mode
Hold down the Command and S keys simultaneously.
See a complete list at Apples official site.