Problem:
This happend to me one weekend when something ran away and exhausted available memory and the OOM process started fighting with the apache process (OOM would kill a httpd to save memory, apache would notice and start a replacement). I had to hard-reset the system, and when I started back up I see this in the logs:
Jul 10 07:00:48 europa imap[21599]: DBERROR db4: PANIC: fatal region error detected; run recovery
Jul 10 07:00:48 europa pop3[21603]: DBERROR db4: PANIC: fatal region error detected; run recovery
Jul 10 07:00:48 europa lmtpunix[21605]: DBERROR db4: PANIC: fatal region error detected; run recovery
Solution:
If things are not too to hosed, you can get away with
# service cyrus-imapd stop
# su - cyrus
$ cd /usr/lib/cyrus-imapd
$ ./ctl_cyrusdb -r
$ exit
# service cyrus-imapd start
If you are hosed harder, you have to go to backups (see
http://www.redhat.com/archives/taroon-list/2005-July/msg00051.html)
(Notes for FC3)