My Android phone constantly says I have a new text message when I deleted the conversation and now I have no way to view it

So back in February I deleted a conversation with a friend and since February it gives me a notification saying I do but every time I go to it it will just bring me to the screen where you text a new person/number something. I have a Samsung galaxy prime and very little tech knowledge so if anyone could help that would be cool. I have restarted the phone often since then, tried blocking the number and unblock it, and tried disabling the notification and it still will not work.