Hi! Your civilization collapsed because they had run out of food for quite some time. You have to keep an eye on the food and happiness levels.
In the beginning the game is rolling if you just keep watering the farms. But, the people keep reproducing until the food production cannot keep up with the food consumption and eventually the happiness levels will drop. It is then, your task to make the tough decision to sacrifice some of the population so that the rest can survive.
In any case thank you very much for playing!