The first thing that I would try is to install another text messaging app and make it the default SMS app. You can install Google Messenger from the Play Store and then see if the app shows any text message conversations that your current default app isn't showing.
You can then uninstall the app and see if that has cleared the messaging number from your current app.
I've had this same issue from time to time, rebooting didn't fix..however give it a few days and it just goes away in its own. Has happened about 5 times on mine.