After reviewing the battery replacement guide for your phone (and having done that same job myself), I have a question. How well did the battery come out? Did you get the adhesives to come out in one piece, allowing you to simply lift out the battery, or did they break (as they are wont to do) and you had to resort to other methods such as heat or prying to finally get the bugger out?
If the latter was the case, the possibility exists that you may have damaged the power/volume/flash/microphone flex cable that runs under the top of the battery. Here's a picture of that flex cable.
I hate to have to suggest it, as I assume you've glued the battery back down, but you should probably open the phone back up and pull the battery so you can check that flex cable for any damage.
Another possibility that comes to mind once you've verified the connectors you removed are all undamaged and securely plugged, would be that the new battery is defective. You may be able to verify if that's the case by simply unplug the replacement battery, temporarily plug in the original one and see if you can power on the phone at all, possibly with a charger plugged in as well.
Give those ideas some consideration and let us know what you find.