i've seen this on a city after i conquered it, too. unrest was at >100% (mostly due to the +50% temporary unrest from occupation, i also had some 22% from taxes, some 20% from number of cities and the territory wasn't linked up with the capital). 100% unrest means all production and research of that city is reduced by 100%, which translates into infinite build times. the game probably doesn't have a separate text label for this sitation, but "continous" is close enough to "infinite" i'd think 