First try this...
1. Turn the laptop off (not sleep or hibernate but off).
2. Connect and external monitor to the laptop.
3. Turn on the external monitor.
4. Turn on the laptop.
Can you see fine on the external monitor?
If yes, you can see on it, then your attached display or the ribbon cable that attaches it are the problem. You would need to look inside the laptop then to see if there are any problems (damage, dents, pinched area, loose, etc.) involving the ribbon cable. If that is fine, then they display will need replacing.
If no, you can't see on the external monitor either, then your graphics card/GPU are the problem and will need replacing (which may require the motherboard be replaced).
NOTE: Do not open up the laptop to do any work on it if it is still under warranty, as that will void the warranty. In such a case you should contact the manufacturer about a repair or replacement.