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Can you supply the model number of your Dell please?
Also, if it is a laptop, are you able to see the screen better if you shine a flashlight on it?
If you are still able to see the screen with the flashlight, this means is the lamp/backlight for the LCD screen has gone out and needs to be replaced. This is typically a repair for a computer repair shop, unless you are comfortable with opening up the screen and replacing the part.
The part is not expensive and can be purchased fairly easily. Here is one for sale at Ebay:
If you have any other questions, please ask...