Welcome to iMore! Which device do you refer? How old is it? Could it need a new battery? How long did you leave it on the charger? Did you try different cables? If the battery is weak, it may take as long as an hour just to show on screen.
Have you tried a hard reset? Press and hold the power and home buttons at the same time for about 30 seconds or until the apple logo appears. (SHOULD take about 10 seconds, BUT if your phone is locked up it could take a little longer)