Should you use on your smartphone the app WhatsApp and suddenly a chat with a certain contact can no longer be found, then it must have been not necessarily deleted. It is also possible that the WhatsApp chat was just archived.
Once a chat has been made "archived" by the user, this no longer appears in the overview of the active WhatsApp chats.
To undo an archived chat within WhatsApp and set it to active, you must proceed as follows:
Open on your smartphone, the app WhatsApp. In the chat overview now scroll all the way down until you see the button "Archived Chats". Tap on this button and you see all chats that are included in the archive WhatsApp. Tap now at a chat you wish to restore, and remain on it until a pop-up menu opens.
Select here: Unarchive chat
The selected chat can now be found in the Chat list again and can be used as usual to send and receive messages from a contact or the group.