When you upgraded, did you remove the ram or the CPU, or the video card?
If you removed the CPU, did you re-apply thermal compound between the CPU and the heatsink?
If you removed the Video card, did you plug the power cable back into it (if your card has one..)
Did you make sure they are all properly connected?
Did you check the cable between the monitor and the video card?
Does the system POST, or is the monitor dark the whole time?
is the monitor plugged into the wall socket? lol...
Does the computer do ANYTHING when your turn it on?
Fans moving?
LED's actvive?
what kind of hardware are you using?