1) Calender on iPhone goes blank even after selecting the calendars to view. It happened out of the blue.
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Things to try first:

1. Force quit the app.
2. Toggle off/on calendar syncing. This can be found in settings/calendar/accounts
3. Delete the mail account associated with the calendar then re add.
4. Shut down the phone and restart the phone
5. Reset all settings: Settings/General/ scroll to the bottom to Reset/ select "reset all settings". This is the second to last thing to try as you would have to reset a few things. No data would be lost but you'd have to readjust some things like bold text, icon size, wifi network rejoin possibly.