Calendar problems with iOS 7 right after updating the device

Solution 1:

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.

Solution 2:

The initial set of troubleshooting steps might be:

1. Attempt a backup of the device.
2. Power off the device.
3. Check that the calendars are still working by logging into the web interface using a computer while the iOS device is powered off to ensure that the service is up and your account is working properly.

At that point, you can restart your phone and start down troubleshooting if it still is misbehaving.