All of my past and future events have disappeared from my calendar, the only events left in there are birthdays and Facebook events. All appointments etc that I entered manually are gone. Ialso cannot add new events anymore — the + sign is grayed out.

I just noticed this yesterday afternoon when I tried to add an event. I'm almost positive thet were there prior to that! How do I get my events back, and fix it so I can add events again!?