You may notice that your LCD screen powers off or goes into a standby/suspend or hibernate state.
Check for these possible causes:
On many laptops there is a magnet located in the LCD bezel (A) (usually the top-right corner). When the screen is closed, this causes a reed switch in the palmrest (B) to close, completing a circuit. This activates whatever action your operating system has set for when the lid is closed. Commonly, turns off the LCD or sends the system into a low power standby mode.
Figure 1: Diagram shows approximate locations of LCD reed switch and magnet that can create a no video scenario.