Originally Posted by
rogerstg
Assuming it's a quill stem: loosen the hex screw on top of the stem as shown in the photo, align the bars relative to the wheels and retighten. It sounds like the stem was too loose to begin.
+1 to this, it's really important to loosen the fixing bolt first, otherwise you could damage your wheel if you really torque on it. Being able to easily loosen/adjust/tighten quill stems are the only benefit they have over the newer threadless stuff.