the network dropped on ubuntu 12.04 server but came back

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http://serverfault.com – I have a ubuntu server, type 12.04 that serves as a mysql box and intranet server. Without notice it dropped all packets (www, ping and mysql). What logs should i check - the messages log is not in the usual /var/log location, and how can I read the "date" out of what the dmesg log states? no other servers on the same switch was down - and it was down for about 3-5 minutes. (HowTos)