Update: I've gotten the computer to boot normally now (had the main power connector on the wrong set of pins), HOWEVER my monitor isn't displaying anything (keeps going to sleep mode) and my keyboard's lights aren't coming on.
I tried, and same thing... the PC sounds like it's booting up but my monitor isn't displaying anything. Also, I'm noticing the front LED around my computer's power button doesn't light up. The computer only starts up once I re-plug the power adapter back in and the only way to turn it back off is to turn off the power switch or unplug it again. That is, pressing the power button on the front of the PC does nothing.Power off and disconnect the new HDD. With everything back to how it started, try to boot and see what happens.
I mean the connector that leads from the chasis of my case (from the area of where my power button is) to my motherboard.When you say main power, do you mean the Mobo connector?