I was 0.5km off hatching a 10k egg so I left the game on whilst driving to sneak the odd 0.1 of a km when I pick my phone up once stopped the egg had disappeared and the incubator was empty and no new or extra Pokemon were to be seen ..... Why is this
Sadly it is quite common to lose eggs when the game freezes as soon as the egg is about the hatch. It has happened to me multiple times as well as many people here. I do not believe this issue has been fixed in the most recent patch. A good way to check if you actually got the pokemon is to list pokemon by recency and if you see a pokemon that you did catch then it was hatched.
My best suggestion to avoid this in the future is that if you know an egg is about to hatch is to not spin pokestops or catch pokemon and just walk until the egg hatches.
Eggs might seem to get lost but they actually hatch. The difference is that due to the app being frozen, the hatching is skipped but pokemons will appear in the pokedex. Just sort your pokemons by recent or check my journal