After the ring has been paired with a device, it starts protecting the collected data from outsiders with an encrypted connection. If another person gets a hold of your ring and tries to pair it with another device, the app will require the user to perform a factory reset to erase any collected data from the ring memory, before it can be taken into use.
If the app notifies you that it's in Restricted mode, you'll need to reset the ring to factory settings in order to continue. After the factory reset is complete, you can reconnect the ring, after which data collection will begin.
Please note that when resetting the ring to factory settings, only the data from the ring memory will be erased. Previous data history that has been synced to Oura Cloud will not be affected.