I’ve seen this happen a couple of times where one of the database tables count of posts differs from the actual number and all the posts disappear.
Using PHPMyAdmin and go into your WP database. Look down your columns (I think it’s the “Records” column) and see if any of them show corrupt or some other error message. If so, check the box next to that table name, then at the bottom of the table, select “Repair Table” from the select box that says “With Selected”.
If that’s your problem, the above should fix it.