Take to 2d part and type pedit at command line. Select one object and it's says do you want to convert to polyline type y here after that press j for join and select the curve on your part. After done check that it's one part and then try to extrude it. Type extrude and select object it wants height and after that draftangle. Press 0 for draftangle and it should be extruded but if you get an error your curve is either 2 or more objects on top of eachother or it's not closed.