This is a very sticky wicket!
Here are some thoughts.
Personnaly, I would have first replaced the power supply before the motherboard.
With all the things you replaced, a new cpu would have cost almost the same.
Something is preventing the HD from spinning.
Did the repair shop test the computer after changing the board? Is there a warranty on the board? Was it new or used?
Is the power supply compatible with the board?
Have you disconnected the power supply to the computer, then press the power button down for several seconds to discharged the stored energy and to reset the protective device?
Have you choosen the correct power supply? Check this Kioskea article:
Have you tested your power supply?:
Gee, I'm glad I am not in your shoes.