i logged onto my world to find it restarted and spawned at the original spawn point with only a map and all man made structures gone

i logged off of minecraft and restarted my computer, when i came back to go into my world it said that there was a problem loading the world. i closed minecraft and opened it and clicked in the world again and the whole thing was restarted! it was like i just logged into a brand new world. all my stuff was lost and all i had was a map in my inventory! all the buildings i made were gone too! i had so much stuff and now its just gone!