I would reopen the phone and check if all the ribbon cables connecting the display to the phone are correctly seated. You can use the beginning (up to step 11) of this guide to verify what goes where and how it should go there:
[guide|577]
Also, while you're in there, check for anything that may have bent or come loose the last time you visited. Such incidents can cause the phone to short out and refuse to boot.
Lastly, be very careful when you're working on the phone since the battery is still connected and possibly charged. Preferably remove the ribbon cables using your fingers or something plastic. No poking around with metal! :)