Try this and see if you can see the screen.
1. Turn off your laptop.
2. Connect an external monitor.
3. Turn on the external monitor.
4. Turn on the laptop.
Can you see the screen on the external laptop? If yes, then the problem lies in your display or its connections.
If the laptop is no longer under warranty you can open it up and check the ribbon cable that connects the display. Make sure it isn't pinched, bent, torn, damaged, loose, disconnected, etc. If that isn't the problem then it may well be the display itself.
Now if you do not see anything even on the external display then it is probably the graphics card that is needing replacement.