Replacing the rear cover requires detaching a lot of ribbon cables and connectors. The connectors in the 3GS are very sensitive and must be connected perfectly or specific components tend not to work (it happened to my friend's speaker a month ago). You should try reopening the device and checking to make sure that all connectors are properly installed. They often times look connected, but a pin or two isn't actually pressed in to place.
If you verify that all of the connectors and ribbon cables are perfectly in place, try having someone call the phone and swipe across the bottom like you would to answer the phone normally. If it doesn't allow you to answer the phone, then the front display ribbon cable is quite possibly ripped or broken.
Try these out and let us know how it goes. Hope this helps! Good luck!