After updating Minecraft a few days to 1.17.30, all worlds created in creative mode have all sunk below the surface or disappeared, are not visible and it is not possible to navigate or move around. There is nothing there, just a huge hole in the ground where structures used to be.
On one computer I spawn on the surface looking at a huge empty hole in the ground where everything used to be, and on another computer I spawn in that hole below the surface, with nothing there and no way to move.

A huge amount work lost for my son who was understandably crying. How do I get them back?

1.17.30