Scenario 1, cause 4: The motherboard is broken
If you determine that the power supply is working, the motherboard is probably completely dead. However, you also have to check if the backside of the motherboard is not touching the computer case in any way. Check if it is not shorted-out by a stand-off or lose screw.
If you still suspect your motherboard, check for blown up capacitors. If you can spot a blown up capacitors like in one of the pictures below, you will either have to replace the motherboard or have the blown capacitors been replaced by an electrician.
If the computer is still under warranty, no problem, return the computer to the shop so they can replace it. If you have to replace the motherboard, try to find an exact model so you do not have to reinstall windows. Also with an exact model you will be sure all your components will be able to be connected.
Examples of blown up capacitors:
