Viber chat histories can be lost or deleted due to numerous known or unknown causes, from which the following 6 are very common.
1. You deleted the Viber app and all conversations were lost.
2. Viber crashed, so you can no longer access Viber.
3. Viber Chat was deleted due to an iOS upgrade.
4. Viber text messages were deleted before you noticed their importance.
5. A jailbreak, black screen, white Apple logo, blue screen of death, error when connecting to iTunes and many more can lead to Viber messages loss.
6. Restoring to iPhone factory settings will delete all content, including your Viber messages history.