Apple Agenda totally empty and "updating" endlessly
I don't know the reason why this is happening, but I had this once. I think it's connected somehow with error of synchronization, it just gets into loop, and never recovers back.

First of all, try to Force Quit for this app, and then to restart. In some cases it helps.

Second scenario, if the previous one haven't helped to troubleshoot, is:


Open up system Preferences and navigate to accounts  > System Preferences > Accounts
Deactivate the iCloud. (try deselecting calendars, as well)
Restart iCal.


If it haven't, then you need to clean the cache. Instructions described here helped to restore:


Quit iCal/Calendar.
Make backup copies of these files before deleting them. (!important in case something goes wrong)
Delete the calendar cache found here:

~/Library/Calendars/Calendar Cache

~/Library/Caches/com.apple.iCal (if these files exist)


You may try the second scenario, provided by the link above, but in this case, you will lose the not saved events, which were not synced with your account. You additionally would need to do these actions:


Delete the Events directory found here:

~/Library/Calendars/*.calendar/Events
Delete the application's preference files here:

~/Library/Preferences/com.apple.iCal.plist

~/Library/Preferences/com.apple.iCal.helper.plist (if this file exists)


And then to reconfigure the preferences and accounts.