Was your computer off and unpluged form the wall wile you disconnected the Powersuplly and installed the new one? If not you might have damaged something.
Leave your old power supply in the comp for now, unplug it form the wall and reset the CMOS. there is a battery on your motherboard, looks like a watch battery. there are 2 ways to reset the CMOS.
1) theres sometimes a jumper near the Battery, 3 pins and the jumper is always on 2 of the pins. to reset it, take out the jumper and put on the the opostie pins, example - default shorting out pins "1,2". switch it to pins "2,3".... leave it there for about 10seconds and then put it back on pins 1,2... then plug your comp back in and see if it powers up.
2) if theres not jumper next to the batt, remove the battery for about 10seconds, and then put it back in. and then plug the comp back into the wall and try powering up.
you may also wont to make shur you didnt just not comething loose? like maybe you accidently unpluged the power button from your motherboard?