Can't see group chat that I've been invited in

[Ratchet2000]

It all started with the group chat not working at all(couldn't see new messages and couldn't send messages) when last week it worked perfectly. I've tried a lot of stuff and nothing has worked and there is no-one on the internet with the same problem from what I've searched.

[VB01]

I've been invited to a meeting but group chat doesn't work for me.  What to do?