Earlier this week one of my co-workers had a problem after they had crashed out of Civil 3D. When he reopened AutoCAD, he could only open one drawing at a time. Well this was familar, but where to go change the setting back was not. In older AutoCAD versions prior to 2007, there was an area in OPTIONS to toggle Single Drawing Compatibilty Mode off and on. Well, in 2008 this does not exist in OPTIONS. You can toggle this mode by typing SDI at the Command line and change it to either 0 or 1. Setting to 0 enables you to have multiple drawings open (default).
So if for some reason if you have a fatal error in C3D (which BTW is becoming less frequent of an occurance with SP2 applied) and you find that you can only open 1 drawing at a time, then check the SDI setting.