iMac startup problem
The iMac starts up, you hear the welcome sound and that’s where it stops
iPhone 8, iOS 13
Question marked as
First off, work thru the steps at > If your Mac doesn't turn on - Apple Support
Then if you get to one of the screens at > If your Mac doesn't start up all the way - Apple Support
follow the instructions for that screen.
If you can not move forward from there, then run the Apple Diagnostic Test.
as per > How to use Apple Diagnostics on your Mac - Apple Support
To be honest, it sounds like you need to contact Apple Support or your local AASP.