My guess is that the replacement alternator has failed. Was this an oem replacement, or aftermarket rebuild? In any event, remove it from the vehicle and have it tested. Many auto parts stores have a fixture to test alternators, or your local auto-electric shop can do this. If it tests ok, you may have an electrical problem elsewhere, for example a communication problem b/t the alternator and the ecm. Or you could just have a burned connector or bad fuse preventing the alternator voltage getting to the battery.