Calendar on iPhone 4s does not work with iOS 7. It stopped working 2 days ago. It won't open on the phone. iCloud is working fine and so is iCal on my MacPro.
How do I get it to work again?
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Make sure you have a Calendar set up in iOS, either an iCloud calendar (Settings -> iCloud) or another one (Settings -> Mail, Contacts, Calendars). If your only calendars are read-only, like Facebook Events and Birthdays, you won't be able to create events.
In iOS 7 and 8, you can check the calendars that have been set up by tapping "Calendars" at the bottom of the calendar view. If you don't see an iCloud calendar or another one you can edit, you'll need to create one.
The initial set of troubleshooting steps might be:
At that point, you can restart your phone and start down troubleshooting if it still is misbehaving.