I installed spamassassin not long ago on our server, and it worked
great!
Then sometime in the last couple of weeks it appears to have just
stopped working. No incoming e-mails are being tagged with spamassassin
headers anymore.

I restarted sendmail (eh, I know that wouldn't really do anything, but
hey, why not,) and I have in /etc/mail/spamassassin the files init.pre,
spamassassin-default.rc, spamassassin-spamc.rc.
In init.pre I have:
loadplugin Mail::SpamAssassin::Plugin::URIDNSBL
loadplugin Mail::SpamAssassin::Plugin::Hashcash
loadplugin Mail::SpamAssassin::Plugin::SPF

and in the other two files, both have:
# send mail through spamassassin
:0fw
| /usr/bin/spamassassin

Between the time it worked and I discovered it no longer working, I
didn't make any config changes, or any server changes for that matter.
What else should I look at to see if I can get it back up and going?

Thanks!
Liam