iPhone 6 lost all calendar entries...all of a sudden hothing shows up. Any ideas?

We were trying to use a family calendar on Outlook.com and my wife was entering items in her iPhone but they weren't showing up on the Outlook calendar...no problem, except that today all the entries on my wife's phone disappeared. No settings were changed, just gone from one moment to the next. Anyone know what could have happened to them? We're talking many, many work, home and personal appointments, tasks, deadlines that will be impossible to replace entirely. Are these stored on the phone and if so, is there a way to get them back? Any help would be GREATLY appreciated.

Scott