Thoes the instructiosn from MT, hmm? They've changed abit. Just make a backup of all your database files first! Then you should be able to fun the sb dump script on *.db do "db_dump -f *.db" (I'm pretty sure that's right)
Basically you just need to upgrade all of your database files to the new verion of Berkeley. Just to explain what happened, is when the sarge upgrade went through on your server, berkeley got upgrade. And this is a MAJOR downfal of erkeley, they almost allways change the structure of the database when you upgrade. So anytime this happen, you're going to have to upgrade your database.
So berkeley got upgraded, and now it's trying to find a structure in your datbase files that doesn't exist - the result? no database. Comments and the entire backend are down. All of your content is there becuase Mt actually creates html archive pages.
Now, once you've got access to mt.cgi back, and your site is working, do your self a favor. convert to mysql! you've got acces to the database, and there needn't be a nitch in yoru site when you switch over. And it will save you the problem of having to upgrade your database as you have to now.
Also, and I found this one out the hard way, if you should switch to an other host that is running an older version of Berkeley - you're out of luck. At least a year ago when I had this problem, there was no downgrade utility. I personally was stuck manually importing my entries from the archive pages back into mt - and I had to totally recreate my page design.
Anyways, enough with my rambling. the instructions from MT used to tell you how to do the entire DB whole sale, and I'm pretty sure that *.db will work, but if anyone else here knows differently, please comment.
Hope this helps, and good luck.