Last night, mail suddenly lost all my emails, which were stored in mailboxes. What happened and how can I find my lost files? I have the latest MacBookPro, running Lion 10.7.2 all software updates installed

Question

Last night the mail application from Apple lost all my old emails, which were stored in mailboxes, neatly sorted by category.  What happened and how do I find them? 
Details:  I shut down mail to run errands outside my office.  My late model MacBook Pro was on "sleep".  When I returned, I opened the application, and (RATS!) all the mailboxes were missing.  They just evaporated. 
Operating system:  10.7.2 - all software updates installed and current as of 24 hours ago (12-30-11, 6 AM EST). 
ln the mail program, the trash folder is OK and the sent folder is OK. 
Yesterday, I did no maintenance on my mail folders.
HELP! 

Answer 1

1 - the sudden disappearance of my 22 mailboxes, used to sort the emails that I need to file for future use
That was most likely the result of some kind of corruption, as I've already said, not a bug in Mail.  Have you repaired the hard drive with Disk Utility recently?  That wouldn't be a bad idea.
2 - at random times, mail (or the server) says that my password is incorrect and I must reenter (fix:  usually I can quit mail, restart, and the connection is restored without reentering my password)
This is a known issue, where a failure to respond promptly on the server's end mimics what happens when the password is rejected.  There's no need to quit Mail, just cancel when asked to enter the password, take the account back online and try again.
3 - mail seems to resort the mail in my mailboxes at random time.  Several times a day, I must reset the sort to (a) by date; (b) by desending
Never seen that before.  That points more to some kind of possible corruption in your Mail data or preference files.