Check whether it's the screen's main power supply, the screen illumination or the video card.
Try connecting another monitor -- or connecting this monitor to another computer.
Try shining a torch at the screen after (you think) Windows has started. If you can see anything with the torch the screen's illumination or power supply to the illumination has failed. If you can see nothing either the screen has failed or the main power supply to it -- see link:
http://www.tomshardware.com/forum/58596-3-official-flat-panel-repairs-thread
