Hello, Do you mean to LIE flat? Many native speakers also make the mistake of using "lay" when they should use "lie." For example, many people go to the beach and lie (flat) on the sand. When you get time, please review these three verbs: To lie (to tell something that is not true) To lie (on your back) To lay (to place something) Those three verbs also confuse many native speakers. They might say something like: "Send an ambulance. There's a man laying in the street." Correct English calls for "lying."

to lay flat= you are lying down on something (bed,floor etc.) and your whole body is flat on the floor. If your hair is lying flat in just means that your hair has the appearance that it is stuck to your head, it lacks volume.