Hello, have you considered re-creating a new messenger group?
Go to the top right corner once you are connected to the messenger app. You should a pencil in a square. Click on this, and then you will be able to select the contacts that you want to add to a new group.
This won't recover your old group, but you can start from fresh.
You could also try logging into your FB / messenger account on a PC and seeing if the group chat is here.