Help with Mac it has flickering question mark

According to Apple:

A folder with a flashing question mark means that your startup disk is no longer available or doesn't contain a working Mac operating system.

If the question mark is persistent, and your Mac does not start up normally:

1. Press and hold the power button on your Mac for up to 10 seconds, until your Mac turns off.
2. Every Mac has a power button. On notebook computers that have Touch ID, press and hold Touch ID.
3. Use Disk Utility to repair your startup disk. Follow the steps for a Mac with an Intel processor.
4. If Disk Utility found no errors or repaired all errors, reinstall macOS.

Need more help?

• Chat now or call 1-800-APLCARE.
• International Support.

Welcome!

What where you doing before the flashing folder with a question mark appeared?

What year and model is your iMac?

Jack