A part of world disappear and the block you placed on to that piece of land lost its texture

I was playing around with the underwater monument and i came up to the surface and on to land, then a huge part of world just disappeared and when i tried to walk on it, its still there, and when i place a block, its there but lost its texture! 😞 Plz fix, when i was building in another world building a roller coaster and when i reached 50 blocks high the block's texture disappeared! Help, why is there so much generation problems!! (by the way i don't know how to upload a picture!)