Did it just happen when you took it apart? If you haven't yet, try a restore. Sometimes the silliest of issues are hardware related. It could be that the backlight on your particular screen has died, which would be unfortunate. If the restore fails, I'd suggest (since you've already taken it apart) looking closely over the display cable for any corrosion. You could then use some >70% (90 if available) isopropyl alcohol to swab it.
That being said, best of luck.