How can I stop calendar data disappeared from my HTC One V?

[AC Question]

Calendar data disappeared from HTC One V

I have a different calendar app that I use than the one that came with my HTC One V. A couple of days ago, all my calendar data disappeared. When I try to add an event, I get a message saying No Calendars Found. Please make sure there is at least one writable calendar enabled in the calendars screen. My calendar widget is still there and I can bring up the calendar, but I cannot add events. How can I fix this?