The same thing happened to me a while ago and I followed this advice which I found on a guitar website and it worked a treat:
Take a bit of paper or aluminum foil and fold it up into a tiny square. Loosen the offending string, and stick the paper or foil between the string and either the bridge or the nut, depending on where the buzz started. Tune the string back up to pitch and try playing each fret on the string again. If you still get buzz, make a slightly thicker square of paper or foil and try again. Repeat the process until the buzz stops.