Disappeared emails!

Hi, I'm currently using Thunderbird 2.0.0.21 for Mac. Yesterday when I launched Thunderbird, mails for 2 years seem to have been erased from my inbox. While there were close to 6000-7000 messages, it currently shows only around 3500 being there. Am a little worried as I need to look up some of those mails intermittently as they are not backed up on my webmail either. Has anybody else faced such a problem, and could someone please help resolve this?