Screen doesn't come on when I turn computer on

Does your pc appear to boot up, do you hear a single bleep to confirm it has passed its BIOS checks.
Is your VDU in standby mode - ie is the power light either orange or flashing - if it is off then check the power cables and connections to the VDU - if the power light is light, check the vga cable to ensure it is plugged it at both ends and none of the pins are bent or damaged (don't be fooled some vga connectors do not have all of the available 15 pins in them) - if you have another PC or laptop you can plug the screen into it then try that to ensure it works - if it does then i suspect that your video card is faulty. check your manufacturers motherboard specification to see which card it can take and replace it