In response to your concern about your Nokia Lumia 900, we suggest that you refer to the following steps below to turn on your phone:
1) Connect/Disconnect the device to the charger for a few times.
2) Please perform a Reset on your phone by pressing and holding the power button for ~10 seconds.
3) Plug the device to a PC using a USB cable and keep it there for 20 minutes.
Furthermore, if your phone was unable to turn on, we suggest that you try to get in touch with your point of purchase for further assistance.