To see or use a locker in your dream signifies aspects of yourself which you have kept hidden inside. Consider what items and belongings are in the locker. In particular, to dream of a school locker denotes hidden feelings, knowledge, and attitudes that you need to learn and/or acknowledge. To dream of a PE locker refers to physical abilities that you have yet to recognize. You are experiencing some physical limitations.
To dream that you cannot open a locker or that your forgot the combination suggests that you are unsure of where you stand in a particular situation. You feel you have lost some aspect of yourself. In other words, you are on shaky ground. If you cannot find your locker, then it symbolises your insecurities about your role or position in a situation.
To dream that someone else is using your locker suggests that someone can see right through you and your facade. You feel that this person has access to your hidden Self.