This afternoon we experienced a brief outage, during which about half our users seemed to lose their subscriptions. This can happen when one of the many complex systems that power Google Reader experiences a glitch. We work hard to avoid problems of any kind, but occasionally something like this happens. The good news is that no data was actually lost, it was just temporarily inaccessible. Google's systems store data redundantly to minimize the chance of anything becoming permanently lost.
We were able to identify, diagnose, and fix today's outage within an hour, which is the kind of response time that we strive for. We'll continue give quick status updates to problems like this in the future so users who have trusted us with their data can feel comfortable doing so.