My iMac will not start up and shows either a question mark or a spinning globe

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.

This article from Apple will explain how to fix the issue: If your Mac starts up to a question mark - Apple Support.

If you have a persistent question mark on your Mac, please follow these steps to see if you can fix the issue

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Turn off your Mac

Press and hold the power button on your Mac for up to 10 seconds, until your Mac turns off.

Every Mac has a power button. On laptop computers that have Touch ID, press and hold Touch ID.

Start up from macOS Recovery

To start up from macOS Recovery, press the power button to turn on your Mac, then immediately press and hold these two keys until you see an Apple logo or other image: Command (⌘) and R. 

When in macOS Recovery, your Mac shows a list of utilities, including Disk Utility:

Use Disk Utility to repair your startup disk

Select Disk Utility from the list in macOS Recovery, then click Continue and follow the steps to repair your startup disk with Disk Utility. These steps also describe what to do if Disk Utility can't repair your startup disk, or Disk Utility doesn't see your startup disk.

When done, restart your Mac: choose Apple menu  > Restart.

Reinstall macOS, if necessary

If the question mark returns after restarting your Mac, repeat steps 1 and 2 to turn off your Mac and start up again from macOS Recovery. Then follow the steps to reinstall macOS.

Best of luck my friend.