Sony SGPT21 - Manual (Page 131)

The tablet device does not turn on.
Make sure the battery pack is installed in your tablet device properly.
If you press the power button with the touch screen lid closed, your tablet device will not turn on. Press the
power button with the touch screen lid opened.
If you do not use the supplied battery pack or an optional battery pack supplied by Sony, the tablet device may
not work properly.
The battery may be extremely low on power.
Connect your tablet device to an AC outlet (wall socket), and charge the battery.
Your tablet device may be turned off completely.
Press and hold the power button for three seconds to turn on your tablet device.
When your tablet device is using power supplied from the outlet, make sure you have connected the tablet
device and the AC adapter, the AC adapter and the AC power cord (mains lead), and the AC power cord
(mains lead) and an AC outlet (wall socket) securely.