On a thread in the official Second Life forum asking about why there was a grid failure on May 7 (fortunately less extensive than the one on April 26th), Oskar Linden had the following to say:
12 hours ago
These issues were the result of a hardware failure in one of our data centers. The issue was noticed immediately and acted upon within minutes. A key first step is intentionally disabling logins and billing. The second step is recovering the hardware or working around it. The third step is bringing services back online slowly to make sure all is well. This entire process took 30 minutes. Say what you will, but that is a very impressive response time. After core functionality was returned we focused on regions that had been affected and worked to get them stable again.
I know it is frustrating when events like this happen, and we understand the difficulties it is from your perspective. We react as quick as possible to stabilize the grid once events like this arise.
Apparently this was the cause of the scheduled maintenance this week for Tuesday, Wednesday, and Thursday, in which residents were cautioned about certain activities.
Hat tip: Daniel Voyager