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. The Z2.log won't give you many clues. It only shows you the hits to your server, but not the exceptions occured.

Where should I look? How could this be avoided/prevented? You should setup a log file for your zope in order to see what the problem is. I don't know how to do it on windows, but I think this has being discussed before. You have to search perhaps for STUPID_LOG + Windows. However, if I not bad and you're using zope 2.7.x, this procedure was made plattform independent, you only have to modify a config file.

Regards, Josef