Does the beer have a krausen (foamy head)? What recipe did you brew? What yeast strain are you using and at what temperature are you fermenting? Most likely your beer is fine, if you are worried use a hydrometer to see if the beer is finished fermenting. Some yeast strains can finish quite fast, but there are a number of other variables that can affect fermentation periods, so it can be hard to determine the cause. Pitching more yeast into the beer will not restart fermentation, necessarily. The yeast that is being pitched must be actively metabolizing sugars, so that once you add it to the beer it can continue fermentation.