What causes this problem?

[Vasanth31]

 Please help me to resolve this issue...

This is vga cable connected 

[WAWood]

@Vasanth31 

It could be any and all of the following issues:

1) cable fault

2) video chip fault

3) Screen fault

To check 1), you would have to use a different video cable.

To check 2) and 3), you would have to connect a different screen -- after confirming the cable was not the issue. If the issue appears on the new screen, it is a video chip fault; if not, it is a screen fault -- and you would have to replace the monitor.