KMail ate half my inbox!

I was using KMail today, as usual, and suddenly could only see messages dated 8/18/01 and earlier in the main folder I use for generic mail. All later messages were nowhere to be found for that folder. Other folders were OK. What could have possibly happened? There is plenty of space on the drive, there weren't any filters that would have caused that. The messages are still on the mail server, thankfully. What is the easiest way to get KMail to retrieve those messages again, without wiping out the file and starting over? Thanks, -ronc