Wedge a chisel between the sash and the frame and gently and slowly chip away at any paint. Check the outside of the window for accumulated paint and do the same.
Pry the bottom of the sash with a chisel and a hammer from the outside if the window is stuck shut. Place the hatchet under the sash and hammer with the mallet. Continue prying. Try placing a 2x4 block on the frame and hit it with a hammer; the block protects the window. This process should pry the stuck window open.
Rub the tracks of the window with wax, soap or silicone or apply a spray lubricant, once the window is open.
Repeat the process of prying the sash open with the upper sash. Once open, rub the tracks on the upper sash with wax, soap or silicone or apply a spray lubricant.