I was using Fullcalendar to show events from a Google Calendar and it was working perfectly for months. Recently, however, it now shows up blank. I've even substituted the calendar feed to make sure there was nothing wrong with my current calendar feed. No luck. The same problem occurs even on the Fullcalendar site and the downloaded gcal demos. I have no idea what happened. Any suggestions?
In case you're still needing a fix, FullCalendar has been updated (13 hours after Brandon's answer) to support v3 of the API.
Here are some quick upgrade instructions: (visit the link for the full wall of text)
On Nov 17th 2014, Google shut down V1 and V2 of their Calendar APIs, which FullCalendar relied upon. Please upgrade to the latest version of FullCalendar or at least replace gcal.js with this file (will work from FullCalendar v1.5.0 until the latest v1.x). Your own Google Calendar API key is now required. Also, the way you specify your event feed is different! Read below about googleCalendarApiKey and googleCalendarId.
(EDIT) And note that there may still be some minor bugs, if you have a problem, keep up with possible fixes here.