Does anyone know what the error message "Distribution appears dead!" means?

I have several distributions applied to several servers. They are set to
refresh hourly. All of them worked perfectly at one time, but now each
distribution is returning this error message for some, but never the same

For example:
Distribution 1 (inventory policy) reports the error on servers a,b, and c.
Distribution 2 (file distribution) reports the error on servers d, e, and
Distribution 3 (file distribution) returns the error on servers g, h, and

I've checked the KB, but found no reference to this error message.

Thank you for your assistance.

-David Lange