world gets ereased and all my constructions and stuff are gone

 I create a world and build or mine and when i close the game and want to play again, it says error cant download game and then i click the world again and all my things and buildings are gone so i can never really do anything knowing that what i do is going to get erased. I bought a parkour map because i wanted to play it but when i joined the world the parkour map was not there. So i wasted my money on that parkour pack. Help me please.