Traditional sash windows operate via a system of weights and pulleys that keep the sash in position. Weights are carefully chosen to balance against the sash, so that when you open the window, you don’t have to actually bear all of the weight by yourself. Of course, this vastly increased the weight of the window. While you can still get weight-and-pulley sash windows, modern alternatives use springs instead. This means that instead of the weight of the sash being counterbalanced by the weights in the frame, it’s counterbalanced by a set of springs, each attached to the pulley just as a weight might be. A spring-based sash window will come with one spring on each side, with the pair balancing the weight of the sash between them. When one of the springs fails, your window will fall down on one side, and won’t stay open. Now, it might be that it’s the cord attached to the spring that’s snapped rather than the spring itself. The only way to find out is to open up the window and check.