Bicycle Tutor

Hi! I have a question regarding threaded stems. I recently unscrewed the bolt (don't ask me why I did it) and I have trouble screwing it back on. Any help on screwing the bolt back in?

If you completely removed the bolt then the chances are that the "wedge has dropped out of reach.
Remove the stem from the bike and look down the steerer tube and you may be able to fish the wedge out. ( or push it up from underneath - wheel out - remove mudguard).

The other option is the "expander type that is like a "cork". It is spinning inside and needs to grip to tighten up. Screw In the bolt and then lift the bolt and turn at the same time. This will get the expander to grip the stem and allow it to be tightened.

See this for the 2 types:-
http://www.sheldonbrown.com/gloss_w.html#wedgebolt

If you pull out the stem you will see which type you have.