I replaced the screena nd now my phone won't turn on?

Are you meaning that you folded the battery when you tried to repair the lcd? If so, you will probably need to replace the battery. They are pretty durable as long as they aren't tampered with too much.

One thing you can try is to do a hard reset(home and power button for about 30-45 seconds) and see if that makes the device come on. If that doesn't, you will need to start systematically checking for power and video troubles. I would start with that battery...