Find and read the log files.
Many good ideas are already posted, but I didn't see logs. When a problem arises, it is probably logged somewhere. If you are still learning your way around, the log files will give you great ideas about what to study next. If you havn't got an issue at the moment, you may see something interesting to follow up on.
I just had a problem with a networked scientific instrument connected to a Windows machine (what can you do? the instrument makers write the software). The problem had to do with bootp, and the logging option was off. I turned logging on, restarted bootp, and got the clues I needed to fix the problem with a simple change to the hosts file. Without the log information, it probably would have been a while before I thought about checking that.