Moved to Site Feedback & Help
If when you left the conversation you didn't check the "ignore future messages" box, then it will re-appear in your inbox if the other member replies. If the other member still has the conversation you could ask them to reply to it, assuming that option is viable in your particular situation
I pretty sure the Admins don't have any special tools for restoring deleted convos, either, but I'm sure one will be along to either confirm or deny that