Facebook has stopped the support for phones running windows phone 8.1 and older (and also some older Android and IOS versions)
The Facebook messenger app wil no longer work as you have noticed by now.
If you want to use the Messenger again, you have a few options based on the info you provided.
If you have a 635 1gb version you can upgrade to windows 10 mobile. The messenger app is supported on windows 10 mobile.
If your 635 is the 512mb version you'll need a new phone, that model is not supported for upgrading to windows 10 mobile