If you have a dinner candle, simply take the candle to the cutting board and carefully, with the candle on it's side, cut back the wax all the way around the wick with a knife until enough wick exposed to light the candle.
If you have a pillar candle, you will have to dedicate some time near the candle while it is first burning in order to get the wick reset. Dig out the wax about 1/4 inch around the candle wick. Light the candle. When the melted wax fills the well that you dug out, empty the melted wax out of the candle. Repeat this melting of and emptying the wax just a few times. The candle should be good to burn normally.
In the future, the next time you light a candle, with a scissor, cut off any excess wick, always leaving 1/4" of wick left on the candle. The reason for trimming the wick is so that it does not burn too hot, shortening the lifespan of the candle.