If the nut on the top of the fork is tight then loosen it and take the handle bar out and see if the part is on the bottom of it. This part is pulled up as you tighten the nut and thus tightening the handle bar to the fork. The part could have fallen down into the fork.
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If the nut on the top of the fork is tight then loosen it and take the handle bar out and see if the part is on the bottom of it. This part is pulled up as you tighten the nut and thus tightening the handle bar to the fork. The part could have fallen down into the fork.
Tighten the 7/16 inch bolt that goes down into the neck where the front wheel attaches to the frame.