Freaking out because you don't know what's wrong with your Iphone? Read this article and you will find some different options to resolve your problem.
Things you will need:
Option 1: Try resetting it by pressing and holding the Sleep/Wake button (on the top right of the unit) and the Home button (circular button on the iPhones face), simultaneously for at least 10 seconds, until the Apple logo appears.
This should automatically reboot your Iphone and having it turned back on.
Option 2: Hold down the home button then plug the usb lead into the back of the computer at the same time.
This may force your Iphone to start up.
Option 3: If neither of the previous options work, then the problem lies somewhere in the hardware or the motherboard.
You should take your Iphone to the Apple store immediately for a professional diagnostic.