I think I may have a save file here from a game where I lost my capital but it is probably still winnable.
I was playing as Germany in the beta of the fall patch. Immortal, Continents. I did a Liberty opening to forward-settle on my neighbour Morrocco. I was surprised when he did the same thing -- the patch's new AI city locations. I fought him for the land and won the war, taking his capital at about T100. I was beelining Education and my CBs were doing a good job at Marrakech, so I completely ignored Machinery.
Then I remembered that Suleiman, who was quite far away at the other side of the continent, had made a city right beside Berlin. Next thing I see, he has a bunch of Longswordsmen and Trebuchets making a surprise attack directly at Berlin. My position is VERY bad, but I think that with good military skills it is quite possible to revert the situation.