Missing calendar events and can't create new ones All of my past and future events have disappeared from my Calendar, the only events left in there are birthdays and Facebook events. All manually entered appointments etc., are gone.

Also, I 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 that they were there prior to that! How do I get my events back, and fix it so I can add events again!?
Go to Settings and click iCloud and turn on Calendars.

If that doesn't work, while in your Calendar, look at the bottom - "Today," "Calendar," and "Inbox." Click on Calendars. Somehow my "Home" calendar (which is where I save most entries, as opposed to "Calendar" or "Work") was unchecked. I couldn't see any old or future entries, and when I added entries and hit save, the day would be empty. When I re-checked "Home," everything reappeared.

It just wasn't showing "Home" entries. I hope your problem is this simple because I know how you feel like you're losing your mind when something like that happens!