There is a good chance you have backups on iTunes then. If you have not synced since you lost the calendars there is a chance to retrieve the data. It's an all or nothing proposition though. You will turn back the clock in every app on the iPad. Any events or contacts you've added since that backup will be lost.
First, open iTunes, go to Preferences, the Devices tab and check to see if you have any backup files. If the file is from before you lost your calendars, it should be worth a try. There should be a checkmark to turn off auto-syncing. Do this if you're going ahead with the restore.
You'll want to copy any documents on your iPad to the computer, via email or another method. Be sure to go through all your apps and make sure important stuff is saved off the iPad. You do not need to worry about Apple Purchased media. If you lose that (and you should not) you can download it again for free.
Now plug your iPad into the computer. In iTunes right click on the iPad under the Devices list and choose Restore from Backup. Choose the backup.
With some luck you'll have your calendar entries back when it is finished.
There is no guarantee this will work, or not cause you to lose even more data; but it is all I can offer.
Good luck.