You should be able to dump the entire db, re-encode it back to the original, and re-import the DB to get you back to where you were.
Programs for translating from one character set to another are all over the web, and pretty much every language (perl, java) have libraries to do this. Once you've identified the pre- and post- encodings, id' google those encodings along with "encoding library", and download/install/run the encoding program over your data.
Even if you don't know the current encoding, and only know what you want to convert it to, it looks like some programs (eg:
http://search.cpan.org/~jhi/perl/ext/Encode/Encode.pm) can help you out.
The lesson here is always, always, always save old data files... if you *do* find some encoding programs, make sure you run them only on the backed-up data you dumped from the db. It would suck to zap your already zapped data into another format.
Good Luck