I've been there too! The problem might be debris has jammed the wheels.
If your wheels are in good condition, you can probably get away with just cleaning them.
Remove the sliding door from the jamb (the door that moves only). Turn the adjustment screws counterclockwise located at the bottom of the window frame, this will lower the door (sometimes on the outer edge of the door frame). If the window is too heavy, do not attempt this without help. Lift the window out of the frame and set it on a couple saw horses or something sturdy with pads so you can easily clean the wheels. With a flathead screwdriver, gently pry each roller from its pocket. (bottom wheels only). Rollers are typically held in by friction and the weight of the door. If any part is warn or broken, replace with exact parts. While you are at it, clean the tracks too. Spray a silicon lubricant on the tracks and wheels before replacing the door. You should notice a really big difference.
Remember safety first.