@SHERRIffc : I managed to solve it at my end.
I have my “regular” phone that I use to call etc. It didn’t have the Facebook or Messenger app on it.
BUT, I think the phone number was used before to send Apple - or any related apps “notifications” to. So, the phone number was somehow linked to my FB and/or Messenger account.
I installed both apps (FB & Messenger) on this phone.
After I opened the Messenger app on this phone (it’s an Android phone, sorry Apple 😅), it asked me “Make a choice about your Messenger experience”. I clicked “Get started”.
Next screen that opened up, was “Choose your Messenger experience” : two options here :
1. Keep using Messenger with your Facebook account
2. Create a new Messenger account
At the bottom of that screen, choose : “Keep using with Facebook account”
Next screen pops up - and you need to reconfirm there as well.
I tried to open Messenger on my iPad after that, and I have now again access - and it is working again !
Hope this helps 😊