So I just got a city out of an ideological revolt, its been annexed and yatta yatta. For whatever reason, I traded the city back to its original owner before telling it to produce something. Only now I can't end the turn because the city still needs me to tell it what to build, which I can't because it's not mine anymore. I've tried the shift+enter trick to no success. Any help is greatly appreciated!
Well, according to this page it's a known bug.
Some users recommended to perform a rightclick. (I really don't know, how this may unstuck te game :D ) If this doesn't work you should have got the autosaves. Which means you lose the last few Minutes.