Usually a monitor turns white screen when the ribbon cable connecting it gets dislodged, pinched, loose, etc. If the device is no longer under warranty, you can open it up and make sure that the cable is connected correctly and not being pinched, etc.
If that isn't the problem, try turning off the laptop, connect and external monitor, turn on the external monitor and then turn on the laptop. Can you see fine on the external monitor? If yes, then the attached display probably needs replacing. If, however, you can't see on the external monitor either, then your graphics card/GPU is probably going/gone and will need replacing (which may well mean the motherboard needs to be replaced).