If you push Pepper's chest button and nothing happens, don't panic. In almost every case an unresponsive Pepper is caused by one of two things: a depleted battery or the emergency stop button. Both can be checked easily.
Make sure Pepper is fully charged. A charger plugged into a Pepper should have a red light if it is charging, and a green light when it has a full battery. Most other color patterns indicate a bad charger.
Double check the Emergency Stop button. We prefer to check the E-Stop by pushing it down first, then rotating it until it pops up, which typically takes about an eighth of a turn. If your Emergency Stop button spins around and around, it is most likely damaged inside and that would account for your Pepper's not booting up.
If you have confirmed that the Pepper is charged and the E-stop is disengaged but your Pepper is still unresponsive, Contact Support.