If you accidentally delete a message thread, you may be able to recover it. First, check your Trash folder to see if the thread is there. If it is, you can move it back to your inbox.
If the message thread is not in your Trash folder, it may be possible to recover it from your email provider. Contact your provider and explain what happened. They may be able to retrieve the deleted thread from their servers.
There is no guarantee that you will be able to recover a deleted message thread, but it is worth a try if you have accidentally deleted something important.