Over the lat 72 hours we've heard from an increasing number of users who are unable to access Google Calendar either via the web or using Spanning Sync. The problem isn't limited to Spanning Sync users—many if not all Google Calendar users are affected.
There's no word yet from Google as to what the problem could be or when we should expect a fix. We'll post an update here as soon as we learn more.
Update: Google has acknowledged the problem and is working on a fix. (16:32 GMT)
Update 2: We're getting reports from some users that their calendars are now accessible, but Google has again acknowledged that "a subset of users may have difficulty accessing Google Calendar at this time", and is still working on a fix. (20:51 GMT)
Update 3: Google reports that the problem has been fixed. (00:34 GMT)