Locating your log files.
Most but not all logs in Ubuntu are situated in /var/log/ and the folders under that location. To find all logs on you server got to a terminal window and type
then hit enter and all logs on your server will be listed.
Open some of these logs and have a close look at their contents. Particularly have a look at /var/log/auth.log, /var/log/mail.log, /var/log/apache2/error.log and the access and error log for each virtual server on your server. When I say open and look at the logs I mean what you want to do is examine them almost forensically. Familiarise your self with the logs get to know the patterns and variations. Looking at these logs can take quite some time at first but be patient, in a short while you will start to see things you need to investigate further,