Minecraft world suddenly disappeared?

Well I've been playing this pack for over a year, so I had tons of stuff and was really far. Today I thought to myself, because I was curious, I would switch to Creative Mode, just for a few seconds and then want to go back to Survival. Suddenly everything was gone. The landmarks are still there, but now there's nothing where my house was. Of course, all of my upgrades are gone too. How can I restore my old game?

Can people access your world?

No, just me

Has it been destroyed like this or has disappeared like this

Disappeared