My iPhone has a black screen and won't turn on but the sound still works. What should i do?

I tried holding down the on/off button and home button for about a minute and it still won't turn on and my iPhone has a black screen but still makes sounds what should i do. I also tried to fully restore it but it says I need to turn off find my iPhone but i can't see the screen what should i do?
please help!

Added (1). its and iPhone 4 with iOS 7.1.2

Do you let your phone dieat least every two weeks, or have the app cocoppa? If you don't let your phone die, or have the app cocoppa it could be a system error because I had the app cocoppa and it corrupted my iPhone to the point I had to get a new iPhone with my insurance plan but assuming this doesn't apply to you you said you tried the reset/restart option with thehomebutton + sleep/wake button the backlight on your phone may be busted. Try using a super bright flashlight then look at the screen. Also I just found this on the apple site if your backlight isn't the problem.

iOS: Not responding or does not turn on
Symptoms

Your iPhone, iPad, or iPod touch may occasionally stop responding to buttons, switches, or touchscreen input and may exhibit one or more of these symptoms:

Does not return to the Home screen after you press the Home button
Does not wake from sleep after you press the Sleep/Wake button
Does not turn on (or power on) when you press any button
Does not start up past Apple logo
Appears frozen or unresponsive
Is unable to "Slide to Unlock" or "Slide to Power Off"
Resolution

If a single application is not responding or stops responding when it opens, you can force it to close.
If the device is unresponsive or if certain controls aren't working as expected, restart your device.
If the device remains unresponsive or does not turn on (or power on), reset your device.
If there's no video or if the screen remains black, verify that the device has enough charge to turn on:
If you are using an iPad, ensure that it's connected to the USB Power Adapter supplied with the device.
Let it charge for at least twenty minutes, then see if it starts normally.
If there's no image on the screen, press the Sleep/Wake button to attempt to wake the device.
If the screen displays a red battery icon, continue charging the device until the battery is fully charged. Learn more about charging iPhone and iPod touch, or iPad.
If the above steps do not resolve the issue, or the if the screen remains black or shows a persistent Apple logo, try restoring with iTunes:
Connect the device to your computer and open iTunes.
If the device appears in iTunes, select and click Restore on the Summary pane. Learn more about restoring iOS software.
If the device doesn't appear in iTunes, try to force the device into recovery mode, and then restore it.
If the above steps do not resolve the issue, contact Apple.