Ditto on the silicone, spray it, let it soak a minute, and gently work the zipper left right up down forward backwards until it will move independently from the formerly fused) teeth. Then, increase the force and work it toward open and closed if there is room. Doesn't matter which way it goes when it first starts to move again, just have to get it off the corroded teeth. Then clean the teeth and zip in the direction you wanted.
Beeswax or candle wax is a great preventative maintenance, rub on in the spring when putting the canvas up. Then no problems in the fall...