If you are running windows then try starting in safe mode. It could simply be your video driver being in the wrong mode for your monitor. If not what OS are you running and how far into the boot does the system go?
Perhaps you should check for loose connections on the monitor and on the cable connecting into the graphics card. If that doesn't work then download the ubuntu live cd then insert the ubuntu live cd and boot from the ubuntu live cd to see if it's a software or hardware falt. If the screen goes black when booting from the ubuntu live cd it means some hardware will need replacing however, if the ubuntu live cd loads fine then you will need to insert the windows installation cd and do a disk repair.