No boot after Windows 10 upgrade

The Dell OptiPlex 745 is several years old.
It was designed for Windows XP and Windows Vista and not for Windows 10.

When did you install Windows 10 in it?
Is it Home or Pro, 32-bit or 64-bit?
What "Version" number and "OS Build" number is it?

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I bought a refurbished Dell Optiplex 745 tower from the internet with a 250g hardrive 64bit with windows 10. I upgraded it to 8 g ram, a new 8400 ge force graphics card and a quad cpu. It was running terrific until I agreed to a Windows 10 upgrade. Then all hell broke loose. The boot got in a loop and it has been going through the diagnostics over and over. I don't have a previous restore point like the older computers once had.

It was running terrific until I agreed to a Windows 10 upgrade

Click to expand...

What was it running before you agreed to "a Windows 10 upgrade"?

You didn't answer my 3 questions in post #2.

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Yes, it ran great until I upgraded Windows 10.

You haven't answered my 3 questions in post #2, and you didn't answer my 1 question in post #4.
You're making it difficult to help you.

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