Did you drop the phone or expose it to moisture? Have you tried a home repair on it? (that is was the screen broken and you replaced it?) It sounds like the phone is working and just the display does not come on? I would suspect the display then. If it was dropped maybe a cable broke or became detached? If it's a repair gone bad maybe it's just a connection you missed? If it JUST happened, maybe the part has failed and you need a repair. Here's how you contact Apple to schedule an appointment to have them take a look....