It’s my fault not checking the status after upgrading the system, but obviously mysql upgrade on Ubuntu hit some problems and stopped in the middle leave the mysql totally inaccessible, which means, my site was down for two days.
Well, I was really busy on job related stuffs, dealing contextual algorithms while playing with network, but this is not the real cause – I do need a monitoring tool to make sure my site is up and running, though it is not that critical.