I was trying to loosen and heighten the handlebars today on an old style mountain bike. I've loosened the nut too much, well completley removed it. I thought that there should be a nut at the bttom of the stem to tighten and hold in place etc.
I'm guessing that this is now stuck in the front tube, not that I can see it!
Any tips on how I retreive this nut?
Many thanks.

Not sure what you mean by "the nut". Quill stems have a long bolt that screws into either a wedge or cone at the bottom end of the stem. To loosen, you undo the bolt a few turns and hit it with a mallet to break the wedge or cone loose. You should then be able to turn, raise or lower the stem if it's not seized. You may have unscrewed the bolt all the way and the wedge is still in place. In that case, see if you can find the threaded hole and screw it back in and do it the right way. If the wedge has fallen down, you should be able to loosen the stem. If it's seized, turn the bike upside down and spray some PB Blaster up into the steerer, wait and try later.

Have you tried removing the stem/handlebars and flipping the bike upside-down and shaking it?

Thanks, I have already removed the centre bolt and remooved the handlebars. Now when I need to refit them, the nut that I know should be at the bottom of the handlebars stem, the same num that the centre bolt should screw into is missing. I've tried turning the bike upside down, but nothing is dropping out, and it didn't fall out previously.
Looks like a trip to Halfords I'm guessing!!

The wedge is still in there. Use a flashlight and look. Can probably use the bolt to screw into it and use a rubber mallet on the bolt to break it free.

Take the long bolt out of the stem, stick the bolt down the headtube, and fish around in there with it. You should be able to drop the end of the bolt into the "nut" (it's an expander wedge, btw), then thread it on, then pull the bolt up, with the wedge on the end of it.

I take the wheel off and screw the stem bolt from underneath. then tap it upwards.
stops the wedge rolling away across the garage and getting lost

Thanks Griftereck, that worked!!