My engine revs rise, but my speed does not, why is this???

I have a 2000 W reg speedfight 100. It has been trouble free until today....i pulled away from a roundabout when suddenly the revs rose but the speed did not. i let throttle off then tried again but it still will not accelerate past 35 mph. it pulls away from junctions, lights etc, and if i keep the throttle open slightly it will get to 30-35. if i then open the throttle fully it just revs higher but keeps the same speed. i thought i heard a whining noise too, but not sure. i rode it home from work which is 7 miles with no problems apart from the speed! can you please tell me what i need to replace, is it the clutch? the rollers? or the belt?