|A lot of possibilities. Are you sure the motherboard is good? Is everything connected correctly? Could the motherboard be shorting against the case somewhere, perhaps an extra stud standoff beneath it? Are you using compatible ram and cpu? Are the cpu heatsink and fan tightly attached? Is the power supply adequate? Do you hear any beeps when starting it up?|
With the power cord removed temporarily disconnect all the drives and remove any add-on cards. Disconnect any USB devices. Then plug in the power cable and see if it starts up. Do you get the posting or logo screen?