Computer cannot start?

As you say there is Hard Drive activity it means that your PC IS booting up although you can't see it as your monitor isn't working! I would check that your cables are pushed in the back of the monitor fully, and check the end attached to the PC first. After that you would need to pull the graphic card and replace it as that is the logical cause. One other thing to check first although it sounds silly,(it's caught me before) Has the brightness been turned down on the monitor. If it is oldermonitor it might have a small wheel under the front edge which can get moved accidently.

Are there any beeps when you boot computer up?
Check all connections to make sure cables are connected properly
Insert a startup floppy in the a:drive and try booting computer.
Open the case to see if the power supply and cpu fans are working properly.
If you have not made any hardware changes you could have Power supply,motherboard or cpu problem,also the hard drive could be bad.
Check memory to make sure that it has not creeped out of the slot.

Check to see if the power supply switch is working properly or has been reset due to power outage.

make sure your cmos jumper is set to normal.while you are inside your mobo inspect all your cards if they are reseated. properly. then restart your computer.

Since you are not getting any beeps at all (not even a post beep?), most likely, something is failing in the POST. Take everything off the motherboard except your processor, power on and see if you get beeps, if you do, add in your memory and repeat the power on. If you get a beep code, add the video card and try again. Keep doing this until you find the one component that hangs the POST. If you get no beep code with just the processor in, reseat your processor. If it still has no beep code, you most likely have a bad motherboard.

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