Problem: Mac Boots to Circle with Line Through it / Solution: Read this
Rarely, your Mac may boot and it shows a circle with a line through it. What the !@#$%^&*() does that mean, and how to fix it? Then read this.
The icon means that you (or someone else) installed another operating system that your Mac cannot boot into. This can be for various reasons, and none of them are a good outcome, but there are things you can do to get your Mac back.
Reason #1:
You're trying to install a version of MacOS that's not compatible with the current hardware. Such as trying to install an Intel version on an M1 Mac or visa-versa. In that case, you must install a compatible version.
Repair the Disk:
Sometimes the prohibitory symbol shows up due to boot error, or an error after a system software update (automatic or otherwise). This is usually easy to fix.
- Shut down the Mac by holding the Power button down for 10 seconds until the computer turns off
- Press the Power button again to turn on the Mac, then press and hold the Command+R until you boot into MacOS Recovery mode
- From the MacOS Recovery menu, choose “Disk Utility” and select your hard drive, and choose to Repair the startup disk
Reinstall MacOS:
Rarely, macOS needs to be reinstalled on the Mac. Fortunately you can choose to just reinstall the system software itself. You should have a backup available of your data, so that you do not lose anything important in this process.
- Shut down the Mac by holding the Power button down for 10 seconds until the Mac is off
- Press the Power button again to turn on the Mac, then press and hold the Power Button (Apple Silicon) or Command+R (Intel Mac) until you boot the Mac into MacOS Recovery mode
- From the MacOS Recovery menu, choose to Reinstall MacOS and walk through the steps
I cannot stress enough about having a good backup of your Data if you have to do the last idea. The best way to backup is by using Mac's Time Machine. It has gotten me out of some major pickles over the years. For more information about Time Machine, read this official Apple Help Doc.