IMPORTANT: note that sometimes the battery will be completely dead and the use of the volume down plus the power button won't perform the soft reset.
The battery can go dead suddenly for various reasons which I won't discuss here since I don't know much about that.
To revive your phone:
Plug the phone into your charger overnight.
THEN first hold down the volume down button, and then also hold down the power button and keep both pressed. It can take as long as a minute or more for the reset to be performed. The reset is intended to do the same thing as removing and replacing the battery on phones with removable batteries.
If the phone doesn't then come alive stop pressing the buttons and give it a little time. If it still does not come alive it is possible your internal battery failed completely and must be replaced. But you can try the charge and reset a few more times, hold down the buttons shorter or longer, etc. If those don't work get on the forums again and search for more answers before you go to the trouble of replacing the battery.