I lost all my contact addresses et -e-mail in the inbox.?

Since this morning, I looged into my mail box only to find out that all my old mail are gone but the sent mail box still has most of my udeleted mail in it intact. What is going on? Those mail are for business and I don't want to loos them. Whey would my Blackberry has the inbox mail and my Yahoomail does n't? Please assist urgently to solve the problem. Thanks, Armand