The Sidekick is made and supported by Danger, which since April of 2008 has been owned by Microsoft. As such, all of the personal information on the Sidekicks was stored on servers owned and operated by Microsoft. During the last week, Danger / Microsoft had hired Hitachi to do an upgrade to their Storage Area Network (SAN). That's usually not a problem, as the owner of the data (Microsoft in this case) performs a backup of all the data in case of an issue.
Well, something went wrong, and it appears that Danger / Microsoft did not have a backup in place. The result is a catastrophe for Sidekick users. T-Mobile sent out a statement last week explaining the situation, and placing the blame directly on Microsoft and Danger: "Regrettably, based on Microsoft/Danger's latest recovery assessment of their systems, we must now inform you that personal information stored on your device -- such as contacts, calendar entries, to-do lists or photos -- that is no longer on your Sidekick almost certainly has been lost as a result of a server failure at Microsoft/Danger."