The most common reason for windows not to open is that they’ve been painted shut. Start by unlocking them and carefully cutting through the paint using a utility knife where the window stop meets the sash. Bump the top frame of the sash with your hand or a rubber mallet to loosen it. This may be enough to allow you to open the window.
If not, go outside, and insert a flat pry bar between the sill and sash on one side of the frame. Don’t start in the middle, because the pressure could break the glass. Pry upward a little to loosen it, then repeat on the other side of the frame, working each side until it raises.
Once you have it open, spray a little lubricant or rub paraffin wax in the track on each side of the frame.