Firstly, has the iPad been knocked or dropped? This could have damaged the LCD, or disconnected one of the flex cables. This could happen anyway, even without a drop, but less likely.
The other problem it could be is a motherboard issue, it water has got inside, or the iPad has been opened up and tampered with in the wrong way. If there isn’t a possibility of either of these, then most likely a disconnected cable, which can be fixed simply by opening up the iPad, disconnecting the screen, and reconnecting it.
Please remember to disconnect the battery if you try this as to not fry the backlight!