Re: new upgrades, no Power.
I wanted to note that a HDD does not have to be "formatted" to work properly. You can plug in a "Low level formatted" (factory formatted HDD), or a tri-boot (multiple OS) HDD, and the computer will still turn on. In fact, you don't even need a HDD to operate a computer (for ex: a "Thin Client")
Also, og's third comment about re-seating the CPU is a common form of "No power, or turns on for a split second" when dealing with a computer build.