My daughter has a 20" iMac that I passed along for the grandchildren to use and now it boots with a black screen. This is what she wrote: "I tried to turn it on last week and although it sounds like it’s on and gets warm as well, the display isn’t showing anything."

Since then I've had her do a PRAM reset. Well, we think she did a reset. There's no starting chime so she had to time her holding of the keys as though there had been a chime. I also had her go through a Safe Boot but she relies "Nada".

So, I'm going to go over there later today and am looking for suggestions. I have TechTool Pro 6; two versions of Disk Warrior; the original grey disks that came with the machine; the disks for a Snow Leopard install.

Recommendations?

Is the iMac running a new enough version of MacOS so it can do an Internet startup? That should bypass most of the internal operating system so you can decide if the problem is hardware—a bad iMac screen, or a software problem. Any way to connect a second monitor? Yes, difficult to do too much when you can't see what you are doing.

Interesting idea, thanks. I don't recall the OS but I'm assuming it would be El Capitan. I found this link, which explains Internet Recovery.

Nope.

Have you got an external clone you can try to boot from?

No external, although it doesn't seem interested in an external boot. I tried with my DiskTools disk but it didn’t work. And now it wants to keep the disk. Even tried a restart and holding down the left mouse button. It made a couple of 'disk drive' sounds but no ejection. Grrrr.

Your TechTool Pro 6 is antiquated and may be unsafe to use, but IF (and that is a big IF) it has an option to create a ProToGo drive you might try using that to create a bootable thumb drive to see if your granddaughter's iMac. ProToGo is a stripped down version of macOS and might boot where the full macOS would fail and it will run the TechTool analytical routines. Alternatively you can create a bootable installer thumbdrive of almost any macOS following these instructions. It is the equivalent of a recovery drive on a thumbdrive complete with Disk Utility.

Actually, I had forgot that, when I gave the machine to my daughter, I had made a bootable thumb drive. (Sorry, artie, for the earlier misinformation). However, the thumb drive does not boot either. Because I could hear the machine running, I took a flashlight and tried to see if I could detect the dock behind the glass (thinking failure to backlight) but I couldn't see anything.

Long story short.....it appears that the machine might need a shop visit which is not a logical option, given the machine's advanced age (14 years). I have decided to advance my decision to replace my existing 27" with something newer, so I'll pass the old one to the grand kids.

Thanks everyone for all your thoughts and time....but keep your keyboards close. I'll soon be looking for directions to ensure a smooth changeover from one 27" to another.