If you're talking about the standard box sash window, it's probably the temp/humidity change of this time of year, and the frame has swollen a bit. You can either wait, or start prying off the trim around the frame (the part that prevents the moving panel from falling into the room!) From there, get a craft knife and gently slice down the gap between the frame and the window to get it moving. Once it's out, spray with silicone spray, make sure the weights on both sides of the window are still tied, and then like the Haynes manual says, refit is the reverse of removal
