Are you saying your return ticket has been cancelled and no alternative flight home offered? I don't think this is the case but I'm confused by you saying there are no longer any seats available for you on the flight, which doesn't seem right if your ticket hasn't been cancelled.

Or are you saying that just your seat selections have been removed and you are struggling to rebook new seats? Possibly because you're no longer silver and no longer have the right to book seats in advance for free?

It's not entirely clear from your post what's actually happened.