On Starting my iMac displays a circle with a bar across on a black screen and stays there

See the instructions in If your Mac starts up to a circle with a line through it - Apple Support

and

How to repair a Mac disk with Disk Utility - Apple Support including the following

However, if your Mac doesn't start up all the way, or you want to repair the disk your Mac starts up from, open Disk Utility from macOS Recovery:

(further steps are in that support article)

See the following Apple Support article for information on a Mac that starts up with circle with a line through it: If your Mac starts up to a circle with a line through it - Apple Support