You need to expand on what exactly happens when you attempt to power up the computer. Do the fans start? Can you hear the hard drive going? Do the power lights come on at the front of the computer? Does the power light on the monitor come on? Did you only replace the motherboard and fit all your other original hardware (RAM, hard drive, optical drives), or is everything new? If you only changed the motherboard, you will probably have to do a reinstall of the operating system.
Can you get into the BIOS at all?