I just built a computer (not sure why, just wanted a spare computer to play around with.) I plugged in all the cords, plugged the monitor and computer into the wall, and pushed the power button. The power light came on, the fans spun, and... nothing. The monitor just showed its little red, green, and blue striped self test. It also didn't beep (not sure if it's supposed to.) My monitor was working properly, I should check the computer power and cable. I checked both, and both were OK. But the computer didn't do anything. I unplugged it and looked inside, everything was connected correctly. So I plugged it back in and, for no good reason at all, a spark flew out of the wall and half of my apartment's power shut off. After resetting the circuit breakers, I plugged the computer in again, and got the same results as the first time (no spark.) My monitor is working fine, it's the same one I'm using on a different computer to type this message.

sounds like there is a short somewhere. when you mounted the board did you make sure you didn't leave any lose screws in the case? is the MBO is mounted off the case and seated on the studs correctly? is the power supply new or used? and is the wire you are connecting the PS good (not freyed or broken?)

er... um...
I accidentally bought an AT case for my ATX motherboard and couldn't return it. So I wound up retrofitting the motherboard in, putting some sort of plastic thing I had wherever the motherboard might accidentally touch something. The PSU is used, but I originally bought it new.
*oops*
I just realized I didn't put an internal speaker in the computer. No wonder it didn't beep... I'll post back with the results if I can find one to put in.

It didn't beep, even with a speaker in it.

it could be a short in the machine. it could also be a wiring fault in your home.

also try a new 'kettle lead' cable and a new VGA cable. by the sounds of things either your VGA cable is dead or your graphics card is either not seated properly or is dead depending on whether you have onboard or add in..

errmmm......

Did you replace the on/off switch in that AT case with a momentary switch? AT cases use a different type of power switch, which is not suitable for use with ATX motherboards. If you haven't got a suitable switch to use, connect the power lead front panel connector header to the RESET switch of the AT case