Start problem After dowloading montery

Did the computer have the Required Empty Space Before attempting the upgrade to Monterey ??

Empty Space Requirements  >> If upgrading from macOS Sierra or later, your Mac needs 26GB of available storage to upgrade. If upgrading from an earlier release, your Mac needs up to 44GB of available storage. 

Apple Authorized Service Provider and / or Visit a Genius at an Apple Store

Therefore, we do not have the On-Hands experience the User ( you ) has with this computer.

Understand, we are all working Remotely from all around the world and there are limitation to what we can do

Did you already try to start up in Safe Mode, as explained in this article?

How to use safe mode on your Mac - Apple Support

كيفية استخدام الوضع الآمن على Mac - Apple الدعم (الإمارات)