Contacts are designed to be synced to a supported application on the computer or with a cloud service.
Sync them back.
If you've failed to use the device as designed, it is possible that your contacts still exist in the backup that was created prior to the upgrade. Restore the device from this backup.
If they miraculously reappear, do yourself a favor, sync them to a supported app or cloud service and start using the device as designed.
This will save you in the future should the device simply fail, become lost or stolen.