I have found from experience, that if you are using the HTTP Proxy in Transparent mode, the WebUI will become unavailable if Squid or DansGuardian goes down.
From the console, or via SSH, run "sudo /etc/init.d/ebox squid restart"
Also - patience. I have found my eBox takes a looong time to fully come up after a restart. I am hoping this will be improved in 2.0
Kind regards,
Geoff