My iphone 4 screen is completely blank but calls will come through?

Yesterday after my mum phoned me when i was walking to school my phone sort of froze and wouldn't let me enter any apps. A few minuets later the phone went completely black i thought it was because it was on low battery, but i have been charging it throughout the day and it makes the noise when i plug my charger is plugged in but won't light up and like i said calls will come through and the ringtone will go but you can't see the screen? I know for definite that i haven't dropped it. What is wrong and how do i fix it?

Do a hard reset. Hold the top power button and the home button for 20 seconds, or until the apple logo appears

It may have just crashed. Try a hard reset - hold both the Home & Power buttons for about fifteen seconds.

That should force a full restart of the phone. It does not erase anything.
If it won't do anything after a few minutes, press the power button for a couple of seconds to try turning it back on.

If that does not work, all you can do is visit an Apple shop and get it checked.
It should be under warranty for two years.