Google services are some of the most reliable on the web, but even Google can't guarantee 100% uptime. Many users (including most of us on the AP team) are seeing an error message when attempting to access Google Calendar online. These things are bound to happen roughly once in a blue moon, but it's always a bit of a surprise when they do.
So far, Google hasn't released any official statement, but Calendar's app status page reports a service disruption which is currently being investigated. We'll be sure to update with more information as soon as it becomes available.
Aaand it's back. Google hasn't said what caused the outage, but it looks like Calendar is up and running once again.