From what I can see it is only a small part of my world, but it has disappeared. Only an area has disappeared and I can see to the bottom where the void is.

My world did fail to open the first time and then when I tried the second time, this is when it happened.

My world isn't slow or anything, but I will fall down it. 

I have gone off Minecraft and come back on again, but nothing changed, not sure what to do.