I don't know why EA or nobody is helping with this question!!
I had the same issue and found a thread that didn't have any answers.. Can't find the thread again to help those that are stuck!!
In order to resolve this, in my situation I had close off Sim City since I didn't want to either roll back (in case I lost a ton of stuff!) or start it again, so I just closed sim city down. Then I was unable to get into my city again due to that message.
It stayed in 'processing' for ages and seemed like it would take forever.. (These maps can't be all that big in file size, right? They're not like Sim City 2000 maps that are MASSIVE!!!)
To fix the problem, instead of clicking 'resume' at the title screen, click on 'play' , then go to the region view over there and you should be able to see the state of your city. There, you can request a rollback. The roll back should not take it that far back.... So you should be good! This should fix your problem!