If you can still faintly make out the app icons on your home screen, it sounds as if the backlight on your screen has failed. If this is the case, a screen replacement would be necessary. Do you live close enough to an Apple Store to go there? If so, make a Genius Bar appointment to have a diagnostic test done on your phone. This would confirm whether or not the backlight itself has failed.
Is your iPhone still recognized by your computer? If so, back up your phone before going to the Apple Store.
If a screen replacement is warranted, Apple will tell you how much it will cost. However, since your iPhone is as much as 5 years old, have you considered upgrading if the iPhone does require repair?