Why does my Mac Pro not booting and why it's freezing during work?

Hi, thanks for the reply seller told me that he tried few ideas and neither worked so he sold cause didn't have time and patience to find the solution. I uploaded photo and video, but can't find the way how to post it in comment it only posts it as words video (1224) for instance but not video or photo.

Here's how to do it:

在已经存在的问题里加入图片

WHAT ideas did he try? We need to know what he has done to it.

I asked seller, he said he changed paste, he swapped graphic card, neither of those worked. They had no time their designer needed new computer so they bought new and sold this one. They didn’t try anything else. But I also tried to change replace battery on motherboard, tried to every possible combination with ram stick and modules, everything was the same. Tried smc reset and I tried nvram reset neither of those worked.

So he's been into the CPU and we don't know if it was reassembled correctly. If it were in my shop I would simply swap the CPU trays to verify it. If you know of someone else with a 20-09-2012 MacPro, that would be your first choice. Lets see if we can identify that loose screw, please post a photo of it. Pull and examine you CPU tray and see if you see any empty hole or any burned components or other anomalies.

I don’t know anyone who is using Mac Pro same as mine, I uploaded photo in my question, you can see there is a screw missing in upper part of tray, at least 1 if not more.