Outside temperature is a BIG factor in starting glow engines. Especially using low nitro fuels. Low meaning below 20% in cold temperatures.

You may want to warm you engine by placing the aircraft indoors for a while. Also, like everyone else has stated, check the condition of your glow plug. As soon as you can, pick up an OS #8 plug. It's hotter than the one you are using and will assist in keeping your plane running. You should also warm up your fuel by placing it indoors and letting it come up to room temperature. At zero degrees, the oil or nitro could have gelled, making it unusable until thawed.

After you get the engine started, leave the glow driver on for a little bit as the engine warms up. Also check the condition of your glow battery. Extremely cold temps, will drag them down quite quickly. You may want to check into purchasing a glow driver that accepts a 1.5 volt dry cell battery for more voltage to your glow plug.

Best of luck.