If the calendar is associated with an email account like iCloud, Gmail, Yahoo, Outlook.com, etcetera then more than likely your calendar data is still intact online. Try the following AFTER you've logged and verified that the calendar data is intact via a computer's web browser. If the calendar data is NOT present in the online calendar then you need to contact that particular service and see if they can undelete or retrieve it for you. If the data is viewable online, do the following:
1. Reboot your device just in case the problem is a simple glitch of some sort.
2. After the device reboots, check the calendar app to see if your data is intact. If so, great, but if not, continue.
3. Tap on "Calendars" at the bottom of the screen and then select "Show All Calendars". Afterwards, select "Done" in the upper right corner.
4. If your calendar data is still unavailable, go to Settings>Mail, Contact, Calendars and then tap on the email account containing the calendar and make sure that the little button next to "calendars" is pushed to the right and displaying a green background. Afterwards, check your calendar for its data.