Welcome to Android Central! Was there any damage to it, like from a drop or from water? How old is the phone?
Try leaving it plugged into a wall outlet for at least 6 hours undisturbed. Then press and hold Power for about 30 seconds. Does anything happen?
You weren't keeping your important data backed up or synced somewhere safe on a regular basis? One should never rely on their mobile device as the sole storage for important data, since in life, s*** happens.:-\