27" iMac mid 2017, no power

My mid-2017, 27" iMac shows no power when the button is pushed.

It was running great until I updated from Catalina to Big Sir.

During installation only went about 25% and the screen froze.

Tried everything in the book, and tried with a very knowledgeable

service tech from Apple. Could not come up with a solution.

So I made the very big mistake of taking it to an Apple store for service.

After getting it back they said they have no idea what is wrong.

After taking it home and plugging it in it had no power, nothing. They said they

did nothing except got it to a point where there was no power and said come pick it up.

Apple store tech said it was because I updated the ram from 32G

to 64G. One comment was because it was not Apple ram it caused the problem.

However, it ran fine for 2 years after updating. Replaced hard drive and nothing,

no power, don't know where to go from here, any ideas, tried reinstalling original ram but nothing.