You can either produce the candle from "thin air", or you can change a silk into the candle. To do the latter, you will need a 12" silk, preferably the same color as the candle.
Produce this candle from a number of places. For example, if you have used mouth coils previously in the performance, gather up the paper coils into a bunch, with the closed candle secretly in the middle, and pop the candle open from inside the bunch of paper. You can also grab the closed candle from inside a production box or hat that is obviously too short to have held a long candle, but when you pop it open, it will seem impossible.