|Is the power led on the computer lighting up?|
Can you hear that the CPU fan and power supply fan are spinning?
Is the hard drive activity led blinking like it usually does while booting the computer ? (If you normally need to logon to Windows, it will stop blinking shortly after the boot gets to that point.)
If those things are all okay, does your desktop computer have both onboard video (built into the mboard) and a video card in a mboard slot ? If so, you will probably get no video from the video port for the onboard video - the one in the same I/O area as most of the other ports on the back of the case. You must plug the monitor into the video port on the card in a mboard slot.
"Do you see anything during the POST, start up cycle?"
POST = Power On Self Test.
All computer's bioses go through a sequence of background tests - a POST - while booting in the very first part of booting - if those tests are successful you hear a single beep from the computer's mboard. Do you hear a single beep?
If you don't, do you hear a different pattern of beeps ?
If you have video while booting, you will see some text and possibly graphics on the screen before Windows (or whatever operating system) loads - do you see anything ?
In some circumstances, you will see video while booting BEFORE Windows loads, but there will be NO video AFTER Windows loads - is that your situation ?
No video is a common symptom of a malfunctioning system. Usually there's nothing wrong with your monitor or the video adapter - something else is wrong.
E.g. Failing or dead power supplies are common.
When they are failing, leds may come on, fans and hard drives may spin, yet the computer will not boot all the way. The hard drive activity led either does not come on at all, or it comes on and stays on.
Have you just been messing with the ram ?
Installing ram that is NOT compatible with using it in your mboard, or a poor connection of the ram in it's slots, can also cause it to NOT fully boot.