If you fly faster than chunks can load and go too far over the blue void, they will stop loading but continue unloading until you are left with a single visible chunk. This chunk remains editable but in walking mode you will be stuck on it as no further chunks will load.
Fortunately, this can be corrected by saving and quitting then reloading - nothing is lost, the chunk loader just looses track of your position and stops loading chunks.
Duplicate of MC-4930.