Sounds like a loose or damaged LCD ribbon cable or backlight cable. Look closely next time the LCD goes blank, if you can see the desktop very faintly, then it is a backlight problem. If not, try pressing the lic close switch near the base of the LCD a couple of times to see if the LCD comes back on. If that doesn't work, try tilting the LCD at different angles. If it is still under warranty, you should return it for service. Never try to repair a laptop yourself unless you are willing to throw it in the trash if you make a mistake and destroy something. If you checked the suggested items and are willing to open the laptop then check for these problems.
Backlight problem (barely visible desktop on display)
1. Check lid close switch
2. Check wire connection to backlight (usually 2 wires from circuit board next to LCD)
3. Replace inverter board (board the 2 wires are connected to)
4. Replace LCD (some people say you can replace the backlight but it is very easy to damage the LCD in the process)
If no display is seen at all
1. Check video output with external monitor
2. Switch video output using Fn + function key combo (usually F5 or F8 on Dell) to see if video comes back on.
3. Open laptop and reseat video cable connections
4. Reseat video card if not soldered to motherboard
5. Replace video cable
6. replace inverter
7. replace LCD
Hope this helps.