Remove the entire barrel and hop unit from the gun. Thoroughly clean the barrel, unit and bucking. Soak the bucking in dish detergent, swab the inside of it out, rinse it off and let dry over night. Swab the unit out of dirt and grease. I assume you are competent and this didn't really need explaining, but it's the first thing I would do. I also assume you can put the assembly back together correctly.
If this didn't fix it, then it could very likely be the extreme cold (unless you were playing indoors). Very cold weather can certainly effect the hop-up rubber and how it effects the BB. I personally saw a handful of gun's hop-up units not working properly last weekend in the cold.
Was this just out of the blue? Did you change ammo or do anything to your gun since last time it worked properly? How long ago did you use this particular gun?