iMac crashes

Monterey 12.6 is the current macOS version.

What year model iMac and what macOS version was it on before the update?

see > Identify your iMac model - Apple Support

During the upgrade installation, the Mac will cycle thru the loading process several times before going to the complete bar and then to the desktop. During this time, you need to be patient to not power down or disturb the installation process in any way.

If you are up and running, but still having issues after the update, I suggest using Safe Mode follow by a normal restart.

see > How to use safe mode on your Mac - Apple Support

Then get EtreCheck and post back here with your report, as suggested by rkaufmann87.

It will help us get a good idea of what is causing your problem if you provide a report of your system, that way we do not have to play 20 questions with you. The report we are requesting does NOT provide any personal information and is extremely safe to use. Please navigate to www.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.

When you have your report, you can attach it when you reply to this message and we can then review it and help you determine what is needed to get your system running well again. 

For instructions on how to download your EtreCheck report and attach it to your reply to this message please click How to use the Add Text Feature When Post… - Apple Community