Because, recently Debian had a PAM UPGRADE (with extended additional functionality) on testing(Lenny and of course SID before that) that forces any service that uses PAM to completely restart. Nothing works, unless you restart the service(s) affected.
With that said, the Debian upgrade should have notified you that "XXX XXX XXX" services need the restart.
It may not have included your needed service.
A reboot SHOULD definitely take care of this (being the big hammer it is)
Ubuntu also had a recent similar issue, but was more of an ABI bump, that was unavoidable to take care of an local exploit.