Boot = Powers on, fans spin, case speaker (if connected) will beep indicating you have passed POST. POST is Power On Self Test.
When you say "cannot boot", what part of the above did not happen. We know you don't have a display on the monitor. Have you checked to see if you installed the GPU firmly into its socket (you should feel it snap into place; it feels the same as installing RAM).
This next tip might be a pain, but use the troubleshooting guide (the link in my signature) to ensure that nothing is out of place. Especially important things to check are power cables to motherboard and GPU.
Lastly, there is a section at the end of the troubleshooting guide that goes into detail about breadboarding (build the computer with motherboard out of the case), this helps test for shorts. Try this too.