If your Mac starts up to a circle with a line through it - Apple Support

A circle with a line through it means that your startup disk contains a Mac operating system, but it's not a macOS that your Mac can use. 

If you can't reset your Mac login password - Apple Support

Find saved passwords and passkeys on your Mac - Apple Support

why does it show a stop sign - Apple Community

How to fix support apple.com/mac/startup

Use Touch ID on Mac - Apple Support

How to back up your iPhone, iPad, and iPod touch with your Mac

Black screen on MacBook Pro? Here's how to fix it

MacBook Pro shows circle with diagonal li… - Apple Community

Stuck on circle line logo after disabled unsupported gpu. : r