This same thing happened to me today. I solved it by uninstalling and reinstalling the app which made me register again and then it worked fine.
However I think I know the reason for this behaviour, whats app got disabled in my main phone 1020 just after my secondary phone was charging (this one also had installed whats app under the same account).
The secondary phone had the chats available, so I knew it wasn't a problem with the service or something weird going on on WP.
Then when I registered again in the 1020 the 2nd phone stated that it was no longer registered. This tells me that there was a conflict between the two phones and one account and thus the 1020 got disabled.
Maybe this is what triggers the problem, maybe it was just a coincidence.
cheers