I have a headless server running 12.04. On occasions (after several weeks of uptime), I am unable to access the server (via HTTP or SSH) -- the connection times out and cannot route to the server. I hard-reset of the server solves this -- after a hard-reset I able to once again access the machine via HTTP and SSH.

Once I have access to the machine again, what log files should I look at to try to diagnose what is causing the problem?