It could be a few things.
First try turning off the laptop, then connect an external monitor, turn on the external monitor and then turn on the laptop.
Can you see fine on the external monitor?
If yes you can see fine on it, then the attached display or the ribbon cable that connect it are the problem.
If no, you can't see anything on the external monitor either, then it is probably the graphics card/GPU that is the problem and will need replacing (possibly needing to replace the motherboard as well).