Hi,
You can try to disassemble the device and to reconnect the Volume/Power buttons and the LCD flex cables.
Use this manual, step 3, pic. 3
Under the battery there are two flex cables. The yellow one is for the volume/power buttons the the small board in the bottom (mic, antennas, USB connector, etc.).
The black one is for the LCD. Both of them are connected to the main board, under the main camera. Try do disconnect and reconnect them. You don't need to remove the battery, only disconnect it.
[guide|54263]