Amazon has provided their take on how the big derecho storm that hit the Eastern US (and still leaves millions without power during a heat wave) brought down one of their data centers. Basically it was “hardware failure” — in this case a couple of emergency generators.
In the single datacenter that did not successfully transfer to the generator backup, all servers continued to operate normally on Uninterruptable Power Supply (“UPS”) power. As onsite personnel worked to stabilize the primary and backup power generators, the UPS systems were depleting and servers began losing power at 8:04pm PDT.
Read the AWS statement for more detail.