[Zope] Zope server stopped today. Why?

Hi, I manage a small Zope/Plone site for our lab, and this morning the server stopped, and I would like to know why, and how to prevent this in the future. We run: Plone 1.05 Windows "standard install" package on a Win2k machine. I looked in z2.log and there was no activity between the last restart of the server (4pm this morning for the nightly backups) and this morning 9pm when the service just was not running. Where should I look? How could this be avoided/prevented? Grateful for any tips! Goran