how could my viber messagelog dissapear? i was chatting on viber and then left for a couple of hours and when i were going to write a message again i saw that all of my previous messages had been deleted except for two of the latest ones. I havent updated or anything, what happend? can it update by itself and deleting messages?
There is no situation in which Viber messages spontaneously delete themselves. Viber does not update without your permission - and updating does not delete messages.

If this occurred on Viber for Desktop, please ensure that the computer that you use is a private computer and no one but you can access it. Additionally, you might consider placing a screen lock on your phone so that your applications are better protected.