Have lost 4 months of emails suddenly... what do I do?

Check for an archive, you might have OE set to auto archive. Also that's what backups are for.

How do I check that? I avhe emails form 2002 but the 4 months of 2007 have vanished...

Look up Archive in OE help.

I just have compacted all folders... is there somewhere we can find the "lost files"

OE stores mail in a DBX file that represents each folder. A DBX is like a database. Each e-mail is stored as a record in the database, so there arenm't separate files. By compacting you made it even harder to potentially recover. If the mails are not archived and not backed up, they are probably gone.