Help! My Imac wont boot up

The flashing ? mark means the computer cannot find the startup volume. Please click If your Mac starts up to a question mark - Apple Support and if that does not help that means you need to take your computer to your local Apple Authorized Service Provider to be serviced. To begin locating an AASP in your general area please click AASP Locator

Happened to me and my 2019 IMAC. Couldn’t recover. Machine still under “coverage”. Called support, made appointment for me at local Mac store (closest Apple store is 80 miles away). There told it will cost $159 to reload system. Says my coverage which was still in force won’t cover this. Amazing that Apple takes no responsibility for update that bricks computers. My old PC somehow always managed to handle updates and when there was an issue it was able to recover without an additional computer or a service call,.

I've already outlined what you need to do, please contact an AASP in your general area.