I just downloaded the latest Mac upgrade and now my iMac is frozen. It won’t even start.

If you can boot your Mac into recovery mode there are three options you can try.

You were asked if you were installing a service patch to your existing OS or attempting to upgrade to the latest OS, Monterey, but you did not answer and so the answers following may differ slightly depending on which OS you had previously installed and attempted to patch with a service update or which superseded OS you were attempting to upgrade from - to.

Use the web links below to lead you to the relevant instructions.

1. If you can boot into Recovery Mode and have a Time Machine backup you can restore your Mac to the state it was in before you attempted to install the upgrade.
2. From Recovery Mode, If you do not have a Time Machine backup, you can reinstall the current OS only while keeping your data and programs intact.
3. From Recovery Mode you can run the First Aid tool from Disk Utilities which may be able to repair the problem with your startup disk.

The following link describe the key combination used to boot the Mac into recovery mode:

https://support.apple.com/en-gb/HT201255

This link describes how to use the various features in Recovery Mode:

https://support.apple.com/en-gb/HT201314

This link describes how to use First Aid in Disk Utility to check your boot disk for errors and attempt to repair it:

https://support.apple.com/en-gb/HT210898

If you do attempt to recover your Mac from a Time Machine backup or reinstall the OS note that the procedure can take hours and the Mac should not be turned off and restarted during the process, even it is appears to stop responding, allow at least five hours of no apparent activity before assuming that the recovery process is not going to work and aborting the process.

Hold down the power button until it shuts down completely. Unplug it from the back and wait about 4-5 minutes. Plug it back in and hold down the power button for about 5 seconds.

Also, when posting please be more specific. We are not there and we are all aweful at mindreading so when you say things like "I just downloaded the latest Mac upgrade ..." we have absolutely no idea what that means. Had you said, " I was running Mac OS Monterey 12.0 and downloaded 12.1...." then we would know.

Only thing I can suggest is to attempt a system reset, which they should have tried in the store TBH, but maybe you already tried that? If not,

Hold the power button in until the Mac powers down, then unplug the Mac and wait at least five minutes.

Plug the power lead back in and push the power-on button then immediately hold down and keep depressed the four keys:

Option + Command + P + R until the Mac boots up and on Macs with sound you hear a second "Chime", for Macs without sound release the keys after about twenty seconds or when the start-up scroll-bar appears.

If it boots normally following that procedure shut down again and then see if you can boot into recovery mode and re-install the OS.

Regarding platform upgrades, the only upgrades you should install are those offered by the Mac OS itself via its software upgrade facility in System Preferences, never download and install new OS's independently, that way you can't be caught out installing an unsupported OS.

If you search locally you can usually find an independent Apple service engineer, we have had to use them ourselves many times to repair old Macs that the official Apple stores will no longer provide parts for. In your situation they may be able to remove the hard drive and reformat it before reinstalling the original OS, the only downside, apart from the cost is that all your data will be lost unless you have a Time Machine backup to recover from.

Sorry, can't think of anything else.

Exactly which update did you download, that machine cannot go past 10.15.x.

Sounds like the hard drive failed or some other hardware failure. Please take it your local Apple Authorized Service Provider to be serviced.

If your Mac doesn't start up all the way

What year is your Mac and what type of storage does it have (hard drive, Fusion Drive, SSD)?

Jack