If you lost or accidentally deleted a chat history with a user, to get it back, just go to the user's profile page and write him / her a new message. After that, the chat will reappear in your dialogs.
Additionally, you can just go to your βMessagesβ page and locate your βHidden dialogsβ. Just select a chat you would like to unhide, click on the three dots and pick βUnhide chatβ from the drop-down menu.
Desktop version:
Mobile version: