WhatsApp chats disappeared after Android Operating System Update
The only one that can help for this kind of issue is WhatsApp. You may want to contact support@whatsapp.com (Source: https://www.whatsapp.com/contact/) and see what they can help you with.


  I just can't afford to lose my three years' important data.


This is the root of your problem. Any important data that you can't afford to lose shouldn't be stored as instant messages, you should have copied over any important data elsewhere.