How can I find out why my EC2 instance was down for a while

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http://serverfault.com – I have an instance running at Amazon EC2. I just checked the monitoring, and I saw that the server was down for a while. TO be more precise, there is completely no line between 15:16:00 and 15:46:00 in the monitoring graphs of the EC2 console. I also have confirmation from Uptimerobot that my servers where down. Apparently my server was down for exactly 30 minutes. I have gone trough nginx logs, and the system log, but I could not find anything out of the ordinary. Everything works just fine now. Can I find out what happened somehow, it is really strange. This is what happened to php-fpm. (HowTos)