iMac (intel) not booting up after unexpected restart

If you are using a wireless keyboard...?

Dig out, borrow, buy an inexpensive USB keyboard and use it to startup in macOS Recovery Mode.

If that doesn't work...?

Then it sounds like a total drive or other hardware failure.

ref: If your Mac starts up to an Apple logo or progress bar - Apple Support

"If your Mac continues to start up to an Apple logo or progress bar, it needs service. Get service"

coxorange wrote:

Good idea on paper, though I've never opened a Mac before and I'm not very confident I could fix it.

Yeah' they are very difficult to work on, plus we're really not sure that is the problem.

You can review some DIY guides and videos, by searching "2015 27" iMac repair"

Couldn't an external drive replace the failed one inside?

Yes if you have the ability to startup in macOS Recovery Mode to erase & install macOS on the external drive.

Or perhaps if you have another older Mac that can erase & install macOS Monterey on the external drive.

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

I suspect a drive failure. At 10 years old it's obsolete and cannot run current version of Mac OS. Consider replacing the old girl with a new Mac mini and a new display.

Not likely, it does not make any sense spending money on it as its value is very low. Apple has free recycling programs you can use. Just click Apple Recycling Programs and find the program for your area.

If you intend to get a new Mac just get one with the same or more storage and memory as the old one and use Setup Assistant and Migration Assistant to migrate your data to the new one. Many 27" iMac owners are now going to Mac Mini's and then getting external displays. Any size display will work, they range from <$100 to about $1600. However for about $300-$400 you can find a very nice one.

After the migration you will be up and running in minutes and will be very happy.

PS Don't forget Apple has 12 month same as cash financing so if $$ are an issue that may be something to investigate.

coxorange wrote:

Thank you both – I'm already using a USB keyboard.

You’re welcome.

Given the age of this Mac, I'm afraid service really wouldn't be worth it at this point (perhaps even impossible).

I agree.

Is there anything that could possibly save it still?

If you were certain that it was a failed internal drive and had the skills, then you could DIY the drive.

However at this point I concur with rkaufmann87. Your best bet is to replace it with a new iMac or Mac mini and external display, then migrate to it using your Time Machine backup.