Have you ever backed up your Viber messages?
There are two backup options at your disposal. One is provided through the Viber app and creates a backup file(sent by Email in form of CSV) with your message data, but cannot be restored to your device. The other only for iOS not Android, through iTunes or iCloud, will create both a backup file of your entire device's data and an option to restore to your iOS device.
If you have, you can get back deleted Viber Messages from either backup.
Download the CSV file from your Email site, but cannot be restored to your phone. (for both Android and iPhone)
Restore your iPhone from iTunes or iCloud back up, which is risky to lose more data. (only for iOS device)
Hope it will help you!
More info about Recovering deleted Viber Messages (better ways):
How to Backup & Recover Deleted Viber Messages on iPhone
Easy Ways to Backup & Restore Viber Messages On Android