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 “Messages” page and locate your “Hidden dialogs”. Just select a chat you would like to unhide, click on the three dots and pick “Unhide chat” from the drop-down menu.
In case you were not able to find the chat with your friend following steps above, please contact our Customer Support Team, and we will be happy to help you by providing the direct link.