System not Booting

Could you please provide the details of your system hardware.

It is much easier to make educated guesses if we know what hardware you have.

If you are lucky, 1 or more devices have slid slightly out of their sockets.

First, make sure that the monitor is connected, power applied, and that the monitor works.

Verify that ALL external cables are properly connected.

If that does not correct things, with power off, open the case.

Either wear an ESD wrist strap, or at least keep the palm of 1 hand against the metal of the PC case.

Make sure that ALL cards and RAM modules are fully seated in their sockets.

Still nothing?
Remove your modem and network cards.

Still nothing?

Look closely at the motherboard for any burnt or charred components. If you see any, you need a new motherboard.

Chas

When troubleshooting, the needs to consider as many clues and other information as is possible to make accurate and efficient diagnosis. Thats why interrogation technique is important and used often. Help us help you by giving as much facts about your system and what you have tried with what results. This avoids the back and forth question and answer.
What is your processor?
How much memory?
Integrated video, PCI, or AGP card?
How many drives including DVD, CD-ROM and hard drives?
What wattage power supply?
Things to look for includes removing the Video Card and examine the edge connector pins for damage also in the slots themselves.
Is the power light on the monitor GREEN or AMBER, heck is the monitor video cable firmly inserted into the computer video port?
Check the monitor cable for bent pins.
You say you observe(lights) and hear(normal sounds) the hard drive booting up?
You can cut to the chase by swapping with another known working monitor since this appears to be a video problem. This will lead you firmly onto a direct path to the problem.
You can also try removing the memory and try booting if we get some beeps then the memory and even the motherboard and power is ?probably? OK. Closer now to a total video problem.
Try removing the video card, plug the monitor into the integrated video on the motherboard, restart the computer and go to BIOS setup, usually the DEL key and set the video option for INTEGRATED.
There is more but answer the questions above and try the stuff suggested.

remove all cards, except video. Remove as much RAM as possible (leaving one strip). Remove any secondary hard drives. See if computer boots.
If not use a different RAM module if available.
If it does boot start installing hardawre one after the other.

If the video card is AGP reseat the card. I have found if the card is unseated slightly it will do this.

Good Luck Lee