If the issue occurred across multiple platforms and brokers, then that is really odd. DOW down 400 is nothing. No idea what would cause multiple platforms/firms to crash, but implies a broad internet issue, or perhaps even some kind of co-ordinated world wide attack on all the servers??? Beyond my expertise.
BUT...if it were just IG....and they have previously refused to have a seconardy server in place to kick in immediately when the main platform goes down, then absolutely they are open to a class action lawsuit, in theory. And frankly, they wouldn't stand a chance.
But I would rather try to work WITH THEM than agaisnt them.