Well are 2 reasons for this behaviour of your notebook...first it can be an issue with the graphics card installed on the motheboard or its an issue with the Monitor cable which is connected between the motherboard and the LCD.
Following are the steps that you can try to resolve the problem:
At first we need to check whether its a hardware problem or not.
1.Restart the computer and tap on the F10 key for a while which would take us to the BIOS SET UP UTILITY...
2.Now we need to wait for sometime in this screen so that we can check the status of the screen whether the screen goes dim or it remains normal..
3.If the screen remains normal for around 10 minutes then its not a problem with the cable,
4.Now we need to get back to the windows screen and try installing the graphics driver for the notebook.
Let me provide you a couple of drivers for the notebook.
I am providing you the drivers assuming Windows Vista installed on the system
if you have an ATI card installed, the install the below mentioned driver:
if you have an NVIDIA card install the below mentioned driver:
or you can install the drivers directly from the manufacturers website..
Hope this works.