Sounds like the backlight for the LCD went dead. Not much you could do... if you're a Sprinter (like me) and you have insurance, go to a local Sprint store and have them put in for a replacement. If your phone has no liquid or physical damage and it seems to just be a faulty backlight, it will be free and will arrive in 3-4 business days. That could tide you over until the new WP8 devices are released on Sprint in the coming weeks. If you also have an older Sprint phone laying around, you could just have your service swapped to that phone until you're ready to upgrade. Otherwise, there's not much else you could do.
If you DON'T have insurance then you'd have to pay $50 for that replacement. Even if you do have insurance and if liquid or physical damage is found (other than your backlight issue), you'll have to make an Asurion claim. There will be a deductible for that, even if you have insurance.
If you'd rather just upgrade now and not wait for a new WP8, that's your only other option.