There are many reasons why your computer screen becomes black. The first and the common reason is moisture. When a computer is kept in humid temperature, there is the tendency of moisture condensing and this collects inside the monitor.
If the video card or the graphic is defective, it may turn the screen black. Connect a new monitor to know if graphics card is a problem. If your screen is not appearing black, graphic card is not a problem and if it appears black, then repair or get a new graphics card.
Black & Decker Air Station
This is not a common reason, but your computer screen can appear black when there is any fault in the RAM. The RAM modules, which are bad, prevent your system from booting properly. It is not easy to know this problem, so the RAM has to be installed in other system to know it is faulty.
Another reason may also be due to the loose connections for your screen to get black. Check the two connections -- the one between the monitor and the main power supply and the one between monitor and video card. Check that all the connections are connected properly.
If the motherboard of your computer has experienced a short circuit, it may make your screen black. If this is the reason, you have to get another motherboard to replace the previous one. It happens in rare cases. To avoid this problem from occurring, see that all the connections with the motherboard are proper and not loose.