Have you reset the device since the update (hold down the home and power buttons until the Apple Logo appears). This often fixes some post-update issues.
When you first opened the Notes app it should have asked you if you wanted to update to the new notes format, and warned you that the Notes would be incompatible with other devices and Macs until they too upgraded. Is it possible you upgraded on another device first, but are trying to access it on the iPad without doing the upgrade?
Since I've updated both my iOS devices, I'm not sure how to check this. Perhaps in Settings > Notes?
Other than that, I have no good guesses.
If you updated using the OTA (Over the Air) method, you can try upgrading again using a computer and iTunes. This is the preferred, more reliable method, and we usually recommend it for major iOS revision updates. Especially for older devices.