Like he pointed out, the issue you are describing is either a problem with the graphics chip, the display, or could be a total system failure. You will need to talk to dell and have the defective piece replaced.
Your problem was in the title "its a dell". Return to manufacturer, your laptop is broken. Screen problems are an indication of graphics subsystem failure or panel failure. Either way, you need to have it serviced.
Like he pointed out, the issue you are describing is either a problem with the graphics chip, the display, or could be a total system failure. You will need to talk to dell and have the defective piece replaced.