Chunk issue?

Greeting!

Upon loading her level this afternoon, my daughter's level has large blocks missing (something like 16x16 minecraft blocks at a time) missing. These went right through her house, through mountains, etc.

We have never used online / lan options at all and after a few weeks of playing in creative mode we haven't seen anything like it. 

Of course, we are n00bs so please take that into account. 

Thanks!