AFAIK, there's no chat history kept and no way to view it even if there was. Chat is a bit hit-and-miss at the best of times. Unless you know the person's IP address (bear in mind, it might not be the same as it was yesterday) or their username on the forum (in which case you can send them a private message), I don't think there's any way of contacting them again.
If you were downloading from them and started the chat, you could possibly find them again by searching for the same file again but it's needle-in-a-haystack stuff really (unless there was something unusual about the file concerned & you could recognise it again).
Hope I haven't confused you too much!! Someone else on the forum might have more clues.
Good luck!