It sounds like the problem is that your Dell Latitude's screen doesn't give out enough light.
As for possible solutions:
1) Many laptops have Screen Bright / Dim keys on the Function keys (F1, F2, etc). There is usually a symbol on these keys to indicate which one does this. Usually you have to hold down CTRL or a FUNC (function) button on the bottom of the keyboard while pressing the F key.
2) You may have a setting within Windows that is dimming the screen as a power saving function.
3) The back light on your laptop's LCD screen panel may have failed. Chances are, it's more cost effective to replace the laptop in a case like this.