It's a universal aggravation for homeowners with aluminum storm windows: You take out the windows to clean them, then you find you can't figure out how to get them back in. A generation of American storm windows seemed to be needlessly complicated in their design, with three or four tracks that various layers of windows and screens are supposed to sit in and metal posts and latches that sometimes retract and sometimes don't--all in a flimsy frame notorious for losing its shape over the years.
Take a deep breath. The window came out of there, so it can go back in.
Raise the lower sash of the window completely.
Hold the storm-window pane on the bottom half of its frame and tip the top edge into the opening. Visually judge which of the three tracks it's supposed to go into based on the width. (You won't be able to just set it in the track because of the two metal posts sticking out the sides on top.)
Tilt the top of the pane downward to the left, so the whole top edge will fit in the opening. Guide the small metal post that's sticking out the top left side into the correct track.
Lower the right top corner and guide the right top post into the track on the other side. Turn the window so the top horizontal span is level and both top posts are in their tracks.
Slide the top of the pane upward in the tracks while moving the bottom in toward the window. When the bottom edge is against the tracks, squeeze the two spring-loaded latches inward on the bottom to retract the lower posts. Press the pane into the track and release the latches.