Macintosh HD has unmounted on older iMac running High Sierra. Disk Utility can't help.

This sounds to me like the internal drive of the iMac has failed in some fashion. "Without warning" is very often exactly how these things fail.

If Disk Utility cannot correct the problem with its First Aid routines, there is still some chance that another utility might be able to shed light on the problem. One such diagnostic app is Drive DX from Binary Fruit.

If you have access to another Mac that can run High Sierra, you can use that to install the OS on an external drive from which you can boot the iMac and continue to use it for your Aperture app.

This is Apple’s guidance on how to Install macOS on an external storage device and use it as a startup disk - Apple Support

Please see this great user authored tip: Use an external SSD as your startup disk - Apple Community (Thanks, Jack-19!)

Sounds like the old iMac's internal HDD has bit the dust.

I see two options if you want to revive and keep using that old iMac.

1) Get a USB 3.0 external SATA SSD and use it as the startup drive for your old iMac.

see > Use an external SSD as your startup disk … - Apple Community

2) Replace the iMac's failed internal Hard Drive, but only if you are skilled at doing computer repair.

It's not easy, but can be done with help from some on-line repair guides and videos.

Edit: Sorry D.I. Johnson I didn't see your reply until after I posted.

Second opinions are not a bad thing, my friend. 🙂

You’re welcome.

If you’re old iMac is a 2012 or later, then it has USB-A 3.0 ports. Which are not as fast as USB-C, but still way faster than using a USB-A 2.0 external drive.