The wind blew so hard that they couldn't play in the park.
Literal
Wind [subject-が] strongly blew [reason-ので], they [topic-は] park [location-で] play could-not.
風が強く吹いたので is the reason clause: 'because the wind blew hard.' 遊べなかった is the negative potential of 遊ぶ. A cause-and-effect sentence about weather limiting activities.