Dropped out spoke

it's not too hard to replace a spoke, but it's made harder if it's a rear drive side one (it usually is as they're tighter) because you'll most likely need to take off the cassette to seat a new spoke, depending on what kind of wheel you've got. a bike shop won't charge much to do it and you will need some specialist tools, but if you choose to go it alone then basically it's like this:

first you'll need a new spoke the same length. if you've got the one that fell out you can measure that, if not there's loads of online calculators (http://www.ctc.org.uk/DesktopDefault.aspx?TabID=3524) or your local bike shop can help.

take off the rear wheel, remove the tyre, tube and rim tape, and cassette too if it's a drive side spoke. feed the spoke through the rim, make sure it goes through the rim and under and over the other spokes the same way as the rest of the spokes that go in the same direction.

feed the spoke nipple into the hole in the rim, and screw it onto the spoke with a small screwdriver. tighten it with a spoke key until it's fairly tight. You'll need to true the wheel, you can use the brake blocks as a guide and tighten or loosen the spokes to pull the rim in the direction you need. It's not a dark art but it does take practice.

my suggestion: take it into the shop for them to do it and ask if you can watch. then you'll have a better idea for next time!