ntpd dead but pid file exists

[unix syzadmin]

Hi,

I see this error in a couple of redhat server.

# more /etc/redhat-release
Red Hat Enterprise Linux AS release 4 (Nahant Update 5)

# uname -a
Linux narheapp017.nielsenmedia.com 2.6.9-55.0.2.EL #1 Tue Jun 12 17:47:10
EDT 2007 i686 athlon i386 GNU/Linux

# service ntpd status
ntpd dead but pid file exists

What could have caused this situation?

Please suggest.
Thanks.