Mac won't restart after update. black screen and color wheel only.

Excellent, please test for a few days. If it continues to work okay then please mark this thread as solved. Doing so may help another user at some point.

I will look forward to hearing back from you.

If your current Mac has a hard disk or a Fusion drive it goes under a lot of strain when a major update is done, there are a lot of read/write cycles that occur. If the drives are marginal to begin with a update can push the HD over the edge. The updates did not cause your problem, you likely have an older Mac that needs mechanical help. However that is all a guess as you did not tell us the year of the iMac or what update you are referring to.

Please take the steps in If your Mac doesn't start up all the way however I suspect your next step is a trip to your local Apple Authorized Service Provider. Apple Authorized Service Provider Locator (AASP) & Genius Appointment

Well a 2019 is still fairly young, if you bought AppleCare and I am right that the HD failed, it would be covered. I hope I am wrong though.

Try this, hold down the start button on the back for about 15 seconds and then let go to see if it starts up. Another thing to try is to startup in the Recovery Partition.

Recovery Partition Startup: Hold down the Command + R on startup. Then open Disk Utilities and run First Aid 2 -3 times and look for errors. If not errors appear then re-install Mac OS and then restart normally. It is possible you got a corrupt download or corrupt install.