This is usually caused by a video graphics card failure. If you have an integrated video socket built into the motherboard, this is serious. Otherwise, just try another card.
First thing to do, though, is to attempt a boot in SAFE MODE. Press <F8> repeatedly once per second as soon as you press the <on> button, then await its boot to safe mode. If it boots to a readable screen here, then your video card is bad... or the driver is bad, or both.