CPU Fan removed and replaced. Can't power on.

Hello,

I have quite the problem on my hands.

The other day, while cleaning up my computer, I removed the CPU fan and cleaned it. When I replaced it, (Exactly as I'd found it.) and hooked everything back up, my computer wouldn't power on. I was wondering if maybe I could have possibly cracked my motherboard screwing the fan back in, or of I've somehow broken something inside the fan itself? I've checked for any other possible problems but there don't seem to be any. The fan was running fine the day I did this, as was my computer. Any help would be very much appreciated.