Good Evening, i have logged on to my minecraft survival world to find that half of my house was missing and fences i built were replaced with dirt so all my cows escaped. is this a bug because i spent lots of time building the house and has all disappeared within a logon. i have saved the house many times before and was all fine so don't know why things have gone. it is a single player server so no one could join and no one has access to my account. please help or fix this bug

many thanks, Tom