Pressure applied to screen makes computer power off

I have a late 2009 27 inch iMac that used to belong to my wife. It died about 4 years ago. Back then, we got her setup with a completely new machine from backup and she loves her newer iMac.

I have been entertaining myself by going through the 2009 machine to see if I could get it working again.

The primary problem was the power supply which I have replaced. After reassembly, the machine would not start or act like it had power at all. However, it would start with the LCD screen removed.

I am now able to get it start with the LCD attached - but loosely. Screwing the LCD screen into place turns it off and prevents restart (it seems completely dead). With the screen loosely placed into the frame and all cables attached - it starts right up (I should mention the iMac does not have an OS loaded - but I was planning to load from a CD of Snow Leopard - and then update as allowed). I get a bright screen with the flashing question mark.

I’m guessing a screwed up cable? But honestly they all look pretty good. So I would have no idea where to begin.

I’d love any advice. It sure has been fun too see the guts and tinker around thanks to all the info here on iFixit.