Well it could be a few things. Try the following to see if you can find the actual cause.
1. Turn off the laptop.
2. Connect an external monitor.
3. Turn on the external monitor.
4. Turn on the laptop.
5. Can you see everything OK on the external monitor?
5a. If no, then your graphics card is the issue.
5b. If yes, then we would need to dig further.
IMPORTANT NOTE: If the laptop is under warranty, before proceeding with any further, you should contact the manufacturer about a repair/replacement.
6. If it is the graphics card, that will need to be replaced, which may mean the whole motherboard needs replacing.
7. If you can see on the external monitor, then you need to open up the laptop and check the connections. Is the ribbon cable loose, bent, pinched, damaged, disconnected, etc?
7a. If it is damaged in any way it will need replacing.
7b. If it isn't damaged, then you should unplug it from both ends and then reconnect them.
8. Once you have replaced the ribbon cable or disconnected and reconnected it, try turning on the laptop as you normally would.
9. If this does not resolve the issue, you probably need to replace the actual display.