If your calendar is associated with an email service like iCloud, Gmail, Yahoo, Outlook.com, etcetera, I doubt your data is lost. Try the following steps:
1. Fire up your Calendar app (I'm assuming you're using the native calendar app) and then select "Calendars" at the bottom of the screen.
2. Make sure your specific calendar has a checkmark in front of it. To be on the safe side, select "Show All Calendars" and then select "Done" in the upper right corner. If your data is visible, great, but if not, continue to step 3.
3. Go to Settings->Mail, Contacts, Calendars and then scroll down to the "Calendars" section.
4. Locate the "Default Calendar" option and make sure that your specific calendar is selected.
5. Locate the "Sync" option and then select "All Events".
6. Go back to the native calendar app and view your calendar data. If it is still not there, go to step 7.
7.Go to Settings->Mail, Contact, Calendars and then select the email service your calendar is associated with and make sure that the button is enabled next to "Calendars". You should be all set.