A few hours ago my Apache just simply stopped responding to requests. It persisted for about an hour and a half until I restarted it and everything immediately started working fine.

The server was not down, the load was not too high and memory usage was not out of normal range. (550mb used out of 720) Apache simply was not responding even though I could see the processes running in htop.

I saw nothing at all unusual in the apache access.log or error.log that would lead me to think it was a DoS or something.

Any thoughts or any other ideas of where to look for clues?

I'm running Apache 2.2.9 on Ubuntu 8.10 w/ PHP/5.2.6-2ubuntu4.1

I'd have no idea where to begin, but as a clarification to someone that may know -- what do you mean by "stopped responding"? Did you get anything (HTTP error) from your server, or did a browser just timeout? Did you try netcat or something from locally, perhaps indicating a network issue?

Like I said though, I'm not sure, but that might clarify.

Browser just timed out... No response from web server at all. I quickly just restarted the service (/etc/init.d/apache2 restart) on it... Didn't take the time to investigate since I have lots of customers on that machine.