I couldn't figure out what had happened.

figure 是动词，意思是"理解、计算、认为"，常与 out 搭配使用。 out 是副词，在这里与 figure 组成短语动词，表示"弄清楚、解决"的意思。 figure out 是一个常用短语动词，意思是"理解、弄清楚、解决（问题）"。