She moved away, and I've lost track of her since then.

lost 是动词 lose 的过去分词形式，lose 的意思是"失去、丢失"。 track 是名词，意思是"踪迹、轨迹"，引申为"行踪、情况"。 lost track of 是一个固定短语，意思是"失去联系、不知道...的情况"。