Google Calendar for Chrome 1.5+ uses the industry-standard OAuth protocol to fetch your calendar. Once authorized, it remembers that you granted permission, and will not request it every time you start Chrome.
Google will be turning off version 2 of the Google Calendar Data API in November 2014. This means that Google Calendar for Chrome will stop working, unless migrated to the new API version 3.
Several users have, in the past, reported problems seeing their calendar events if they had not logged on to the Google Calendar web app in several days. Using OAuth ensures that Google Calendar for Chrome always has the necessary authorization to fetch your calendar events. So if you depend on seeing your events in the dropdown instead of in the full web app, you will no longer see a blank calendar, even if you haven’t logged in to the web app in a long time.
Since OAuth is the only supported means of authorizing Google Calendar for Chrome 1.5+, it will stop showing events if not authorized. It will still remain installed, but will not be able to show any events.
If you see this in the popup window, then you need to re-authorize.