missing startup disk on iMac

Welcome to the Apple Support Communities.

In Recovery Mode, please go to Disk Utility, and select "View" > "Show All Devices". Select each device (or entry shown), and take a photo of it and upload them all here. 

This will allow us to see if your startup disk has failed, is corrupted, etc. and identify the proper resolution path.

Cheers,

Jack

That happened to me recently.

The solution?

Get expert service and support at the Genius Bar.

👉🏼 Call 1-800-MY-APPLE or get help here. 

I was all done in 20 minutes (and the store was busy). 😉