One of my calendars is stuck on my iPhone. I tried to double-click the home button, but the Calendar app is not one of the icons showing in the task bar, so I can't close it manually. Can you assist?
Try restarting the phone. This will close all apps.
Hold the lock button (on top of the phone) for 4 seconds, then swipe the red bar to turn it off. When the spinning icon disappears, hold the lock button for 1 second, and an Apple logo will appear. Try using your calendar again.