If you previously enabled backups, this is the safest and most realistic recovery method. The exact screens may vary slightly by device and app version, but the overall process is similar.

On Android
Make sure you are signed in to the same Google account used for the original Viber backup.
Install or reinstall Viber from a trusted source. If needed, you can download Viber from the official download page.
Open Viber and activate it with the same phone number used when the backup was created.
When Viber detects a backup, choose the restore option.
Wait for the restore to finish before opening multiple chats or closing the app.
After the restore, check important conversations first. If some recent messages are missing, the backup may have been created before those messages existed.

On iPhone
Confirm that you are using the same Apple ID and iCloud account that stored the Viber backup.
Check that iCloud Drive is enabled and that Viber has permission to use iCloud.
Reinstall Viber if necessary.
Activate Viber with the same phone number.
Choose restore when Viber offers to recover your chat history.
For iPhone users, iCloud storage space matters. If your iCloud backup was never completed because storage was full, Viber may not have a usable chat backup to restore.

Check Whether You Have a viber desktop Backup
If you still have access to Viber before reinstalling, check your backup status first. This helps you avoid accidentally removing local data before confirming that a cloud backup exists.

Open Viber.
Go to the settings area.
Open the account or backup section.
Look for the latest backup date and backup account.
If no backup appears, create one now for future protection, but remember it will not restore chats deleted before the backup was made.