You did a hard reset... I assume you did the button push method? Did anything show on the screen during hard reset? Does it show splash pages during boot then go blank?
If yes to these - you may have a corrupt file somewhere. Try one of the reset tools to reflash your OS and Firmware:
FAQ - How can I recover/reset/restore my phone software? - Microsoft - USA
If no to the above questions (the display never shows a thing), then you need to seek warranty replacement or qualified repair.
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