Found the Problem. Solution is in the Main Question
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I changed the Power Supply with no success. Then I pulled the screen off to see Diagnostic LED’s. I then shorted the Power Button pins (named PWR-BTN) on the logic board to test it. So I know the Power Button is working. PowerSupply is also working.
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'''SOLUTION:'''
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CHANGING MY HDD (already not original) with a new SSD solved the problem.
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So the problem was within the Disk Drive with the Boot sectors. I cloned it to the SSD with Disk Utility from iMac.