This happened to me to. I play on Western Europe 3 in a private region with a friend. I had two cities, one of them a 100 k plus university/industrial city the other somewhat larger and refining oil into plastic and fuel. While playing the later I got the message "Your city has stopped working properly" and chose to rollback.
Afterwards I could not get back into the city. I have tried to load it many times, but when I try to load it I get the message "Unable to load the city at this time. Please try again". If I try again the game never finish loading.
After the rollback I couldn't get into the first city either (same region), but was able to visit my friend's cities. Its been about a week, and tonight I started a new city in the same region. From there I was able to enter the first city, which seems to be in the state I left it. I still can't get into the second one, where I got the rollback message. When I hold my mouse over it I can see that it's been claimed by me, but the city itself seems to be gone. If I click on it I can see the population is still 195 k and I have 810 k simoleons. Ambulances, firetrucks and police from the lost city somehow continue to help my remaining city. No way to load the city itself though.
I will probably try to build another city, but I'm concerned I will lose that city as well. If I keep losing my cities, that is (to me) a game breaking problem.
It's been almost a month since this thread was started, and the issue is still not resolved.
Edit: I've tried the repair program, but that didn't change anything other than force me into the tutorial again.
