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. 

Get started with accessibility features on Mac - Apple Support

5 Ways to Turn On a Mac Computer - wikiHow

Use Siri on Mac – Apple Support (UK)

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

Sign in or out of your Apple ID on Mac - Apple Support

macOS User Guide - Apple Support

If your Mac isn't connecting to the internet over Wi-Fi - Apple Support (CA)

If your Mac is lost or stolen - Apple Support

Erased HD - Apple Community

Make and receive phone calls in FaceTime on Mac - Apple Support

Mac Boots to A Circle with Line Through it 🚫, How to Fix?

How to Start Up a Mac in Internet Recovery Mode - iFixit Repair Guide

Help me regarding issue 2106F when I trie… - Apple Community