Hello, Yesterday one of our servers went down and I can't see a reason.
I checked Logs (messages, auth, apache error.log, syslog, kernel) but I didn't find anything unusual on them.

These are the Munin charts: http://html.webporte.jp/munin.html
(Its very clear when the downtime happened)

I would like to ask your opinion on what are the possible reasons (if it is possible to guess though munin charts).

I have my personal opinion, but I want to hear from you.

Thank you in advance.