Under this condition, we have to ask help from professional recovery software. Jihosoft iPhone Data Recovery is no doubt one of the best. You can view the recovered messages for free with this tool.
1. Install Jihosoft iPhone Data Recovery.
2. Run the program.
3. Connect your iPhone/iPad to computer via a USB cable.
4. Click “Scan”.
5. Click “Viber” to view the messages in conversations after the scanning is done.
Related: Retrieve deleted WhatsApp messages on iPhone