I couldn't help thinking about what had happened.

couldn't 是情态动词 can 的否定过去式，表示"不能"或"无法"。 help 在这里是动词，意思是"避免"或"阻止"，常用于否定结构中。 couldn't help doing 是一个固定搭配，表示"忍不住做某事"或"情不自禁做某事"。