lost world

Me and my friends have created a minecraft world either in november or december and have been playing on it since. Last night it had been working perfectly and when i get on the next morning, it has a grey picture as if it was deleted and is now 500(was 300) mb. I click on the world and it tells me syncing world and proceeds to kick me out of minecraft. when i load back in the world is no where to be found, i am quite annoyed and want to retrieve the world back as quickly as possible if it even is possible.