Hmm. A site admin account can only be disabled by another site admin, or when your Cloud subscription has been cancelled (either by asking or by not paying). But you should have been emailed about any cancellation if you're the owner of the Cloud systems.
You don't have admin rights if you can't re-activate users on that users "edit user" page.
I haven't currently got site-admin on a Cloud system to check this, but I remember some talk about "if a user is completely deleted from a remote directory (or atlassian account), you still see their account for the history, but can't re-activate it because there's nothing there any more". I'm not sure if that was just a suggestion or if it had been done. That could explain it too.
Hopefully! Last time I ran into it, Atlassian got in touch about 36 hours after I tried to log in and got that message. I didn't have to contact them, and better, they didn't actually ask me any questions, just said "we needed to do some verification, we've completed it, please log in now".
I can't do anything about verifications - we're a community of end users who try to help each other, not Atlassian themselves (although you can find them here). Your user will need to contact them directly to chase it up! I would go via https://support.atlassian.com/contact
Update: That's a wrap! Thanks so much for your questions. If you didn't make it for the live AMA, not to worry. Add your questions below and I will get to them ASAP. Hello Atlassian Community...
Connect with like-minded Atlassian users at free events near you!Find an event
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no Community Events near you at the moment.Host an event
You're one step closer to meeting fellow Atlassian users at your local event. Learn more about Community Events