Description is too general. Too many variables. Type of device, is the phone on but screen is black, phone is off but the screen is black, water damage, phone was dropped, battery issues, screen issues, on and on. Under warranty talk to carrier, vendor. Not under warranty 3rd party repair shop.
I'd suggest you call your job and ask them if they can replace it for you, I don't know how your job handles that, best of luck.
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