When the deadline was moved up, the entire team started to lose their head.

lose 是动词，意思是"失去、丧失"。 their 是代词 they 的所有格形式，意思是"他们的"。 head 的意思是"头部"，在比喻用法中可以表示"理智、头脑"。 lose their head 是一个习语，字面意思是"失去他们的头脑"，实际表示"失去理智、慌乱"的意思。