Open WhatsApp
Go to the Chats screen
Scroll to the bottom of the chat list
Tap Archived
Find the chat you want to unarchive
Long press the chat
Tap Unarchive
On iPhone, swipe left on the archived chat
Tap Unarchive
On Android, long press the archived chat
Tap the unarchive icon or Unarchive
To unarchive automatically when a new message arrives, go to Settings
Tap Chats
Turn off Keep chats archived