losing worlds and freezing

ok so all of a sudden my mine craft starts freezing and it deleted two worlds i WORKED very hard to build!!!! this has made me very mad. i buy alot of worlds from yall and i am tierd of trying to build and then it delets them freezes and every thing i worked hard on is gone!!!! i have cleaned my computer and every thing i possabilty can do.So its not on my end.i have decided to get rid of this damn game!!!