1. Turning off your calendar:
This simply means at some point you "clicked" your calendar and the color disappeared from your list. An easy way to notice this is if the square color beside your calendar is not colored in. verses
2. Hidden Calendars:
This is where you'll want to click the drop down arrow beside where it says "Other Calendars" and go down to Settings. From here you'll want to make sure the calendar you're looking for is checked off as "Show in List". If it's unchecked it won't show up on the left hand side with the rest of your calendars.