Handlebar of my bike will not bolt onto the front forks I decided I'd like to remove my front brake from my bmx, but having looked closer i realised I was looking at a gyro brake system. I proceeded anyway, thinking this wasn't going to be a problem, and I removed the front brake with ease, although having to remove the handlebar to get the cable out. I then went to reattach my bars to the frame using the same long bolt that was in there before, but it just wouldn't screw into anything inside the tube. What did I do wrong and how can I fix it?
Sounds like a quill stem, something like this:



That wedge bit at the bottom sounds like its come unscrewed completely.  Take the stem out, and look down the hole.  If you see a lot of light, then you've dropped the wedge and its gone.

If there's only a little light coming through the bolt hole in the wedge, then flip the bike, take the front wheel off, and any front mudguard, and poke the long bolt up from below.  The wedge should fall out the top.  

Once you have a wedge again, loosely bolt it to the stem with the long bolt as pictured, insert into the steerer tube, and reassemble.