Why can't I add events to my calendar? After upgrading to iOS 7, the + (plus) sign on my calendar is grayed out.  I cannot add new appointments. I even tried using Siri to add an appointment and the response is "Sorry, I'm afraid I can't do that."
I had to turn on calendars and set the password in iCloud (settings/icloud/turn on calendars).  Make sure to reset your phone by holding down the power button and the select button at the same time until your phone turns off and the apple icon appears, then release.  Give it a second and it will reset.  It should work after that :)