I have just one thing to say if that can help you out. Please visit Facebook and go to the activity log, if you remember the date or month filter it in the search and there you may be able to do some action. I don't know exactly but I think you may find a way there.
Let me know further.
I did have to message that person on our individual chat, in order to get them to be "un-ignored".
I read that was the way to fix the problem.
Then I thought that would help the group chat, so I wasn't getting those pop-up's.
Until I "unistalled" the app & "reinstalled" the app.
Now I the pop-up message about that person - is GONE! =)
To view your older messages (messenger chat history) please follow these 2 simple steps:
1. Go to Messages
2. Choose the thread of the friend you want to search. Scroll all the way till you find the message you want.
(Optional) If by scrolling you can not find the message, click on Options (the little cog) and then on Open in Messenger to see all ...
Your question is somewhat unclear so these official pages will help you find your answer.
How do I turn Messenger notifications on or off?
How do I choose which Facebook notifications I want to get as text messages (SMS)?
How do I change my notification settings for a Page?
How do I adjust my email notifications from Facebook?
Choose What You're Notified ...