I've now seen three devices loose the entire Calendar. All events are gone, all calendars are gone. It doesn't make any sense. You can tell if this is the case by going to Settings | iCloud | Scroll down find find Default Calendar. If it isn't there then you have run into this problem. The only work around that I know of is you sign out of your iCloud account and sign back in. Remember all of your settings. Best to take a screen capture of all the screens so you can recreate what you had.