Check out [this image](https://www.parktool.com/assets/img/repairhelp/Quill-Stem-Definition.jpg). You likely lost the wedge (labeled here as D). It’ll be somewhere in your bike’s head tube. Remove the stem, pull the wedge out of the head tube, rethread it on the bolt, then reinstall the stem.
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Check out [this image](https://www.parktool.com/assets/img/repairhelp/Quill-Stem-Definition.jpg). You likely lost the wedge (labeled here as D). It’ll be somewhere in your bike’s head tube. Remove the stem, pull the wedge out of the head tube, rethread it on the bolt, then reinstall the stem.