Hello, I'm sorry to hear that. Hopefully I can help you with this.
On the list of WhatsApp chats, does dragging down make the Archived Chats list appear? If you archived a chat, it should appear here at the top.
WhatsApp shouldn't remove an archived chat unless you have unarchived it. And even unarchiving a chat just brings it back into the main chat menu. You would have to actively delete it, which shouldn't be possible from a few accidental swipes.
Check if you have made a recent back up of your chats - if your Grandads' chat is in there, you should be able to recover it.