|Disconnect all drives and try booting again. |
All that is needed to POST (boot) is the following hardware: Motherboard, CPU w/HS/fan, 1 stick of RAM, power supply, graphics card or on board, monitor, keyboard. No other cards, RAM, mouse, cabling, etc.
Only connect the power supply to the motherboard. Main 24 pin connector, auxiliary CPU power connector, any power connectors required on a graphics card. Be sure the CPU fan is connected to that header. Momentarily touch the 2 start button motherboard pins with a screwdriver to start the system. Unplug to stop.
You may have something like a dead hard drive that is stopping the system from booting.
Did you clean off the thermal material on the CPU and then apply new thermal paste according to directions for your processor?