After locating the fracture or crack in the pipe, the first thing to figure out is why the leak is occurring. The joint could be old and the glue holding it to the pipe may have lost adhesion. The pipe may also have moved due to a lack of supports holding it in place, so check that the pipe is well supported to wall studs/ceiling joists, etc. Finally, a lack of insulation and cold temperatures may have caused the problem: make sure there's enough insulation surrounding all your pipes. But whatever the cause, the joint will have to be replaced.
Turn the water off at the main shutoff valve. Cut through the plastic pipe six inches on each side of the leak--use a hacksaw to cut the pipe, and make sure that the cuts are straight. Remove all burrs from both cut ends using a tradesman's knife.
Measure the distance between the two cuts in the existing pipe, and cut a new piece of pipe to that length--as before, remove all burrs. Prime the ends of the existing pipe and new pipe, as well as the insides of two straight couplings--use PVC primer. Wait for the primer to dry--usually a few minutes.
Apply PVC glue to one end of the existing pipes, the inside of one coupling, and one end of the new pipe. Push the coupling up onto the existing pipe, and the glued end of the new pipe up into that coupling--hold the coupling in place for 30 seconds.
Glue the inside of another coupling, and other end of the new pipe, and remaining end of the existing pipe. Push the coupling onto the existing pipe end, and the new pipe into the other end of the coupling--you may have to push the pipe out of position to do this, but there should be enough give in the plastic pipe to accommodate this. Hold the coupling in place for 30 seconds. Wait for the glue to dry--5 minutes--and turn the water back on.
If you are working in a confined space, make sure that your tools/supplies are in easy reach, and that you are relatively comfortable while working in that position (on your back, side, etc.).