Lost 'preview PANIC!!!!

I have been making this quite complicated image of Brighton Pavilion.....I have been working on it for hours.

This moning it suddely went oulines. I saved the image, switche Illustrator off, left it foir a bit, but when I opened it up again and clicked preview I just got the last ratyer pathetic almost white image......HELP PLEASE.

I have used Illustrator foir years but have never had this problem before.....any thoughts?