Add some yeast energizer and nutrient, make sure the ambient temperature is high enough for the yeast to do its thing, and add some more sugar. If you used a plastic fermenter, maybe you had a fast fermentation and the CO2 leaked past the "Tupperware" type seal and you saw little action in the airlock. If that's the case, you don't need to do anything.
Check your SG before doing anything. If it's where it should be for the FG, then you're done!
Don