My calendar won't work on my iPhone 4 since upgrading to iOS8. I am unable to save events All my history and future events (except birthdays attached to my contacts) are gone and I can't save new events.  When I view my calendar in iCloud from my computer they are all there.  They're just not syncing with my phone!  I don't have my calendar in my phone synced to any other calendar either. How can I resolve this?
I struggled with this for quite a while. Out of nowhere, I could not add any events or reminders to my calendar, and the (+) in the corner of my calendar was grey. Super frustrating!

Go under settings. Select Mail, Contacts, Calendars.  You must click on individual accounts, and move the slider for calendar to the "on" position. For instance, I have an iCloud account, and multiple gmail accounts listed. I clicked on each account, and turned on all of the calendar sliders.

This fixed the problem, and I am now able to add events, and view birthdays and events.