[Zope] zopedown

[Ben]

A Zope server stopped responding to requests, with page requests just timing out, but fortunately (I thought) I had the debugger URL ready.  However, when I loaded that I was amazed to see it load instantly, showing the usual 4 threads.   I restarted Zope, and now the debugger shows only:

Threads traceback dump at 2008-04-25 23:30:12

End of dump

In Unix, I see the usual two Zope processes.  Their CPU usage is very low.
 17633 ?        S  0:184640842280  0.0  0.3 /usr/local/bin/python /www/Zope-2.7.3b2-0/lib/python/Zope/Startup/run.py -C /ww
 17632 ?        S  0:00 8192 3768  0.0  0.1 /usr/local/bin/python /www/Zope-2.7.3b2-0/lib/python/zdaemon/zdrun.py -S /www/Z

The event log has nothing unusual, and ends with "Zope Ready to handle requests"
What can I do?