replaced lcd, device does not turn on

Replaced the lcd/digi with frame but device does not turn on. When plugged into a charger, a green light is present letting me know the battery is fully charged but holding the power button does nothing. Holding the power and volume down buttons only turns the light (and the device) off then back on... no display, no vibration or sound. Have checked the connections 100 times, used multiple screens, but can't get any life from this device.