We have virtualmin running on a windows server, without any notice it just stopped working. there was no problem connecting to admin interface but no sites were up running and restart the server would not help. we had to restart the main server and then the virtalmin and then everything worked fine. But why did the server just stopped, no errors in the log and there was no other suspected errors.
Well, if no sites were up, that may point to an issue with Apache. You might want to take a peek at the Apache error log.
Normally I see this issue when the apache logs have grown too large. Make sure both your access_log’s and error_log’s are being rotated correctly.