I have no idea what changes were made, so all that follow is pure speculation. I think that they may have moved their hosting, or at least their DNS hosting to a new server with a new public IP address. This would involve using what's called "glue records" hosted on ICANN's root DNS servers. IF this is all the case, glue records can in my experience take a notoriously long time to replicate the changes out to every corner of the Internet. Root DNS servers have a very high cache time to live (TTL) value set on these, 48 hours. And I've heard (though I've not personally seen actual evidence) that some larger ISP's will have their own cache of root records and setup even longer cache times.
When I was working, our normal procedure for changing glue records was to make sure that the old server would be up and running for at least a week after the change. If possible we would enable logging and look for queries for the domain to stop coming in to the old server before decommissioning them. I've seen it take 5 days or so before queries stopped routing to the old servers, and heard (though again not confirmed) of up to a week.
Again...all speculation on my part as I have no idea what changes were done and know nothing about their hosting setup. Just dipping my toe in the tech troubleshooting pool to test the water and see if I want to dive back in while I start contemplating re-entering the work force I suppose. :tiphat: