I was upgrading an old Gateway tower Pentium 4 1.7 GHz CPU with 3.0 GHz (I think) part, and I was ill-prepared in that I didn't know about the little locking bar, and thus thought it was connected and powered up. Now the fans turn on, but the hard drive is silent and it won't boot up. The question would be: what would be the damage that occurred? Motherboard, CPU?
If you're lucky, it could just be that the new processor wasn't compatible with the motherboard. You do not work with a running system for safety reasons. It could be the processor or motherboard.
I'd try reinstalling the old processor and seeing what happens first. There's a chance its nothing, or the system is beyond salvage. In future, unplug the computer before you do anything, getting shocked, or burnt is a very real possibility. I've personally been shocked, blown up a PSU and a hard drive working on a live system so ya, I can wholeheartedly recommend unplugging the system FIRST before doing any work on it.
There can be pain, destruction and magic smoke escaping.
With modern systems, it is not enough to simply turn the computer off before adding or removing parts. You need to, at the very least, turn of the switch on the power-supply itself, but even better, just pull the power-cord out altogether.
Many motherboards these days have an LED on them to indicate that they have power running through them even if the system is “off”.
What likely happened was that when you were working in the motherboard a piece of metal (screw? screwdriver? cable?) happened to touch some pins or leads on the motherboard that happened to correspond to the power-button or short-circuited the power-supply connector or some other thing that caused it to power up. Obviously this is extremely undesirable when working in a computer.
Based on your description of the current symptoms, it sounds like power-supply has burned out and cannot supply the needed voltage for the hard-drive to spin up. If this is the case, you will need to replace the PSU.
You did not mention beeps, so POST beep codes are not helpful, but this means that it cannot run at all, and since you are not even sure about the CPU, the processor may not even be compatible with that motherboard, causing it to do absolutely nothing. Put the original one back in to see if it works.
However, even if the CPU was not working, the hard-drive should still spin up, so it is likely both.