Either the machine itself is down or frozen, or there is a network problem between the spong server and the machine. If you find that a large majority of machines have their ping service marked as "red", this would typically indicate a networking problem (or a wide scale problem associated with a power failure or some other disaster).
If the ping service is not available, then chances are most services running on that machine are also marked "red" (or "purple"), since if the machine can not be accessed via the network, any client information probably won't be reported either.
If there is a connectivity problem, then a traceroute is done between the spong server and the machine being monitored. This information might be helpful to someone trying to determine where the problem lies (if it is a networking problem).