iMac won’t restart or load up

If your Mac doesn't turn on – Apple Support

You have provided little information about what Mac your computer is. It's important to tell us what year and model you have. Also which OS are you trying to install? Is it the same one? A couple of thoughts come to mind. You say it it is an older computer, but don't say what year it is. You could possibly have a failing hard drive due to its age. If you are using wi fi to download your new OS, it may have gotten corrupted. That happens with wi fi. You might try again to download a new installer using a direct cable connected to your network interface. I think you are saying you ran Disk Utility. Is that correct? If so, did it end with a report that said everything was OK? I not, rerun it until it until it does. If your hard drive is failing, Disk Utility will often so report. Beyond this, I don't know what else to suggest. Maybe someone else has an idea that may be helpful.

I believe that all Macs have a serial number on the bottom. It is my understanding that it's possible to visit the Apple help site and somewhere there it can provide the model and year of your Mac. Incidentally, I have read here that some of the experts have noted that it is not unusual for a "ready to die" hard drive will fail when an attempt is made to reinstall an OS on it, because it stresses the drive.

You have been unable to download High Sierra. Is this the same OS you had before this problem? If so, then compatibility is not the problem. Since the computer was running slow in the beginning, I have to suggest that the hard drive may have failed. I hope not.

Sounds like the Hard drive has died, can you get an external drive & install to it?