okay this happened to me too. loosen your clamps on your bars. then loosen the compression bolt on the fork. loosen it so u can see like 2 inches coming out of your fork. then pit a long axle in ur fork like the way you would put ur wheel on but leave out the wheel. stand on the axle and pull your bars up untill the compression bolt it back against the fork. then tighten your compression bolt again untill it pulls the bars down to the headset.
if it still doesnt tighten and the bars dont come down then you have stripped the threads on your starnut and you need a new one.