The fun part of repair! Doing everything right, then the %#*@ thing doesn't work. It happens.
Start by holding power and home at the same time for at least 30 seconds, or until the Apple logo shows up. If the Apple logo shows up, you're good. If not, connect it to your computer and see if the computer detects it. If it doesn't, do you at least see the battery charging icon? If not, time to take the screen off.
Take a look at the battery cable. Make sure there is no damage. Did you disconnect the battery connector to do the repair? Make sure its hooked back up if you did. If you still don't find anything, take the shield off the connectors and make sure you didn't miss a connector. Take each one off, reseat it, them put the shield back on followed by the screen. Hold power and volume for another 30 seconds or until it boots.
Did it work?