[Zope] Proxy Error

I have suddenly started getting a 'Proxy Error' msg when accessing my Zope site which uses Apache to redirect specific requests to Zope. I can access other virtual hosts running hosted by Apache, and I can get to my Zope site if I specify port 8080, but for some reason just http://www.myzopesite.org/ returns a 500 Proxy Error reason: Host not found Can anyone suggest what is wrong? It would appear to be an Apache error. Can anyone suggest where I look for the source of the problem? -- John