One of the first fixes you'll find on any forum is resetting the NVRAM (nonvolatile random-access memory). It can be any random issue from iCloud not allowing you to sign in to certain apps just not opening. Sometimes specific processes on your Mac just don't work. Reset the NVRAM and PRAM: option-command-P-R Release the N key when you see the choices appear.Start or restart your Mac and immediately press and hold the N key on your keyboard.Your Mac must have capable firmware to boot from a system. Start up from a NetBoot server: Nīooting from a NetBoot server allows you to boot from a network, rather than a local hard disk or optical disk drive. You can also hold option-D to start up from either utility over the internet. Select Apple Hardware Test or Apple Diagnostics.Release the D key when you see the choices appear.Start or restart your Mac and immediately press and hold the D key on your keyboard.How to start up from the Apple Hardware Test or Apple Diagnostics You can run Apple Diagnostics once you think you've isolated the issue to a possible hardware issue. If there are issues with your Mac that you think may be related to the hardware, you can run the Apple Hardware Test. Start up from Apple Hardware Test or Apple Diagnostics: D

Click the arrow or press return on your keyboard.

Release the option key when you see the Startup Manager window.

Start or restart your Mac and immediately press and hold the option key on your keyboard.

So if you have your Mac partitioned to run Windows or a macOS beta, for example, you can select that startup disk instead of the default disk. The Startup Manager lets you choose a different startup disk during boot. You can leave safe mode by restarting your Mac without pressing any keys during startup.

Release the shift key when you see the login window.

Start or restart your Mac and immediately press and hold shift on your keyboard.

Deletes font caches, kernel cache, and other system cache files.

Prevents Startup Items and Login items from opening automatically.

Verifies startup disk and tries to repair directory issues if needed.

Safe mode is a way of starting up your Mac that makes sure it performs certain checks and prevents specific software from loading or opening automatically.

Start up from Apple Hardware Test or Apple Diagnostics.

Here are all the keyboard shortcuts you can use when starting up your Mac! Whatever your needs, you can boot your Mac in a variety of modes to complete a task before startup all you need to do is press some keys down and wait for the chime (unless you have a newer MacBook Pro, sorry). Sometimes you need to boot in Recovery Mode to reinstall an OS or recover your hard drive via the internet. Sometimes your Mac isn't running correctly, so you need to reset the NVRAM and PRAM.

Apple netboot servers