Replacing batteries on 3gs can be quite a pain. I would try a Hard Reset, holding both Power and Home buttons together at same time for approximately 15 seconds until iPhone boots up. If this does not work then….First the battery may not be compatible or functional, and have to be replaced. Second it may just need a charge. Third you may have damaged iPhone installing it. Flip vibrate switch, if vibrates iPhone it is on and LCD may not be connected correctly or is damaged. Connect iPhone to iTunes to see if it is recognized, if so LCD may not be connected correctly or is damaged. If nothing yet, try to charge iPhone with Wall Charger only, maybe for 20 minutes to see if that will get it going. If still nothing yet, carefully open iPhone and check that you put the Logic Board in securely especially over the battery area, make sure Connector 4 is really perfectly set right, and make sure Connector 1 for the LCD is set perfectly when closing the iPhone back up. Good luck. Brad