Worlds That Have been previously saved multiple times deleting themselves

This has happened twice to me now on two of my best survival worlds. 

I have saved both of the worlds multiple times and there has been no problem, but as i saved and exit the world then wait for the menu screen to appear showing it has successfully saved  the world i was just on the world isn't there. so i then tried many things i saw online and none of them helped and i ended up here, Reporting this bug

Please fix this and make sure no one else has this issue

please message me back.