iMac failed to complete power-on boot. No volumes shown in macOS Utilities

Thank you for the reply. I have already tried what you suggest, but the disk utility does not find any error on any item shown in the left column of Disk Utility. Also, there is no icon shown in the UI where the disk or disks should be shown in the macOS Utilities screens. Consequently, I am unable to initiate any function that requires selection of a disk or disks.

Thank you so much for this. I was beginning to to think that it might be a fatal disk error. It's a two-hour drive to the nearest Apple Store for me, but that's what I am going to do. I especially like the suggestions and links you provided. I'm off to buy a new drive. I'll post back tonight or tomorrow after I have gotten back and tried #2. Thank you - if you're ever in the East Texas area, look me up and I'll buy you a beverage of your choice. Joe C.

Well, no luck so far. A five-hour roundtrip to get the SSD, then the first problem when I tried to erase it. The instructions in the link for #2 above said to use APFS format. But this iMac is booting into Sierra (I think it was) and APSF is not offered. So I chose Mac OS Extended (Journaled) instead. The network install is always the choice that runs, and it is very slow. I have tried it twice now, with reboot and disk utility in between. But each attempt ends with an error indicated and I had to start over. Should I use a different format or is the Mac version that is being loaded too old for those instructions to be valid? I think I'm going to see about ordering the mini and display if I can find enough change in the couch cushions. LOL...

That is what I did, or at least was trying to do. But the network install finally fails after running for quite some time. I think I'm going to order the mini and a monitor. Do you have any suggestion on a good 24 to 27 inch monitor that I could run from the HDMI port? Or is there another connection that would be better?

Please disregard my last question about a display suggestion. I just ordered the mini and a display and should have them both by this coming Tuesday. My sincere thanks to you for your help today and I hope you have a great weekend.