World Disappears from world selection menu

I was last on this world two days ago when I exited out like usual. When I got on today I could not select that world on the menu. It was not there. When I check the game files the world is still in the saves file. But when I go back into Minecraft it was still not there. Unfortunately this is the second time this has happened to me; the last time it happened was several months ago. Is there anything I can do to access the world again? thanks!