[CentOS] Machine becoming irresponsive

There is a CentOS 5.2 machine that is sometimes found to be offline. It runs a few websites but nothing very high traffic. I happened to notice a few days ago that before it went down, one of the sites written in PHP was throwing errors that it could not connect to the MySQL backend. Two hours later, the whole server was down and wasn't even responding to SSH.

It's not my box, but I may have opportunity to look at it. After going through dmesg and messages, if I don't find anything obvious, what should I start looking for? What are the likely, common culprits and how to identify them? Is there a page of the fine manual that addresses issues like this?

Thanks.