Hmmmm - Make sure the extra P4 connector is plugged in, basics, things like that..reset the CMOS, chances are you might have done this already.
Might not have been good to give power to your HD without it being hooked up to the motherboard though if you left those hooked while you crossed those wires to let the PSU turn on.
Ummm.....reseat graphics card and RAM, if those don't do it, try reseating the CPU.
If all else fails, take the mobo out of the case, lay it on a cardboard box, make sure the case is nearby so you can hook up the PC speaker.
Leave only CPU, one RAM stick, graphics card and keyboar plugged in.
Pull out RAM, see if the computer gives you a beep code, if it does try reseating the RAM, if it still doesn't boot then the RAM could be the problem.
Same thing applies to the graphics card, just follow the above steps.