| The soundpost is simply a dowel-like piece of wood that is held between the top and back by the string pressure, under the treble side bridge foot. Just get the bass to a luthier and he can put it back up for you, at little or no charge. No big deal; any time you take down the string tension, the post can fall down. As long as you're there, have the luthier show you what he's doing, as it's not tough to do it yourself as long as you have a general idea of where it needs to go... | 
