Hopefully they get some fix over this matter, as I am flying for more than 10 hours now this thing happened abruptly. The whole scenery went black and it won’t get back as per system status, all from weather down to Live server are all checked in green.
Try doing a short haul flight instead
This is a known issue and I believe it is going to be fixed with the coming update.
In the mean time, here’s a message Phillipe posted on a topic about the same issue:
It is likely a connection problem sometime during your flight which caused the scenery to be corrupted. Use the link provided by @Luke to see more solutions to this issue. Good day