MacBook Pro Clamshell mode dysfunctional

Have you tried safe-mode for troubleshooting? {Start up your Mac in safe mode - Apple Support}

What is the MacBook Pro model? {Identify your MacBook Pro model - Apple Support}

Also good to mention the monitor model that is connected.

Depending on your model, have you tried any of the below resets

• • How to reset the SMC of your Mac: Reset the SMC of your Mac - Apple Support
  • Reset NVRAM or PRAM on your Mac: Reset NVRAM on your Mac - Apple Support

I am having this problem as well. 

MacBookPro 13”, M1-2020

Chip Apple M1

MacOS: Monterey 

Ext Monitor: LG 32UD60

Worked as a pair since acquiring my M1-MBP and for several years before that with my previous MBP. I did recently abort an installation of a older version of Adobe Reader (according to document I received instructions (legit)) only to have it get stuck at 75% for days - even after multiple restarts. I read where somebody else had used a hard shutdown (holding down power button until unit shut off) to fix this problem. Thought I’d try it. It stopped the loading, but at some point thereafter my MBP stopped working in clamshell (could have been immediate or could have been days - just don’t recall).  Works fine as second or mirrored monitor just NOT in clamshell - using the exact same cables. Not sure if there is a causal relationship between the Adobe install and the clamshell fail other than being temporal.  Anyone able to help? It’d be greatly appreciated. 

Thanks. 

Remember that the default action for closing the cover on your computer is SLEEP.

ONLY when power is connected and an external display is attached (and likely an external keyboard and pointing device) is the expected action 'closed display mode'.