Always start simple.
Pull the screen, disconnect the battery, then disconnect and clean the LCD connectors. Clean them thoroughly with alcohol. Take a look and make sure you don't have any bent pins, then reconnect everything and plug it in. Hopefully you see an apple. If not...
Hold down power and home for at least 30 seconds. See if you get an apple. Look closely at the screen to see if its just really dim.
If that doesn't work, try your original screen. Does that one work?
If you try all of this and still can't see an apple, then its time to move on to board level stuff.