computer will not start

Hi, is this a desktop or laptop pc?
Do you have the install DVD?
Will it start in Safe Mode - tap F8 as it is starting up?

 

If you are able to, do the following.
Turn the Pc off - remove the plug from the wall.
Once you have opened the case.
Touch the metal case to discharge any static.

Remove any dust - do not use a vacumm cleaner, too much static.
Use a can of compressed air. [Pc shops will have them]
Check that all fans are free of dust and will rotate.

Remove Ram - Video cards etc and then Reseat them in their slots.
But first - clean the copper connections of the Ram - Video cards with a pencil eraser.
If more than one Ram stick - use just one stick at a time installed in your Pc, as a test and see if it will start.
Check all cables and connections.

Put the case back on the Pc - and see if it will boot to the desktop.
Cleaning inside your computer
http://www.howtogeek.com/72716/how-to-thoroughly-clean-your-dirty-desktop-computer/