Make sure the lid is tightly secured to the bucket. If you are using a plastic bucket as a primary fermenter, most of the time the beer is fermenting, but the lid isn't on tight so the CO2 created by fermentation is escaping through the leak rather than the airlock.
The next thing to check would be the gravity reading. If your reading has dropped from the original gravity reading, or the starting gravity reading our kits suggest, then fermentation has taken place.
Another case may be that the beer finished fermenting before you got a chance to check on the fermenter. In our experience, dry yeasts and some strains of yeast can finish in as little as 1-2 days!
If the gravity has not dropped, one option to try is to gently swirl the yeast back into suspension and make sure the temperature is within the optimal range for the particular strain of yeast you are using. Bringing it towards the warmer-end of the range may help kick start some fermentation.