Board Game Arena

A player who is not my friend sent me a message, but I accidentally turned off the chat box and forgot his name. How can I click it again?

Open your profile menu (click on your avatar in the top right corner), then you have the 3rd tab (speech bubble icon) that will list all chats.

I saw that button, but I still can't find it. Maybe he sent it to me through the previous game discussion.

You can access recent game discussions if you go to the table and click "discuss". So you could look through your game history and check some of the recent games that you think might've been it.

The message might also have come through a group chat from a group that you are both in. In that case I think there is no way to access it, after you left the chat once. But in that case, it was not a personal message to just you anyway.