I find my world or parts of it lost when I re-enter my computer.

Its been 3 times that when I re-open the game, I find my house, cave, mine etc lost or moved somewhere else, or I am missing parts of them. 

even myself I have found myself in a completely different place. 

its very frustrating, as I am losing hours to build a world and suddenly its lost and I have to start again....!

thank you for helping!