Basically, for the past two days the main campaign (as well as custom games, scenarios, multi) has been locked for what seems to be the majority of the playerbase. Except for the few who got it sorted, the only people able to play now are those, who bought the DLC campaigns or the expansion. The largest thread regarding this issue on SteamHub has over six hundred replies and the topic on Ubisoft's forums is even bigger.
I'm kinda in no position to whine cause I heard about it before the purchase, but since the game was on a daily sale I assumed they were bound to fix it pretty quick. I was wrong.
Anyway, if you were planning to buy Heroes 6, you should hold off for now.