I use Empathy as chat client for IRC. Since it is the default chat client, I expected that I can update (availability) status from top menu, but it is disabled.
This have happened to me too a few times. What I did was:
Make sure empathy is actually running (there should be a little arrow in the Messages indicator).
Open Empathy and check to see if it have failed to connect to a im network (msn, google talk etc.).
Usually it's the latter.