when I played my mine Craft world it crashed when I opened my mine craft again my world was gone I didn't deleted it!!!!!!

when I played mine craft it crashed in 10 minutes and saw the crash report. when I opened my mine craft agau=in my world that I played was gone suddenly gone All my hard work was gone I didn't deleted that world but I coulde find it again, just that world was gone !!!!!!! Please help me I !!!! please help me find back my world !!! I had to alll start again!!!!!!