Import flash.display.Sprite (1046:type not found)

1

ive been working on a as3 project using flex and fcs3 for a few months now and have had no problems. now im trying to get a new project started using flex and fcs3 but now i get this errror telling me that import flash.display.Sprite (1046:type not found). how could this be? ive tried running my old project and it still works.