Post a photo of the front end of your bike.

What type of a stem do you have? Quill Stem vs Threadless?
https://en.wikipedia.org/wiki/Stem_%28bicycle_part%29

Quill on top.


Threadless Below

If you have a quill, you use the large nuts below the stem to tighten the headset. If you were tightening the bolt on top of the quill stem, perhaps you broke it. Your bike would be very dangerous to ride with that bolt broken.