Sony SGPT11 - Manual (Page 125)

The tablet device does not turn on.
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.
When your tablet device is sitting in the cradle (sold separately), make sure you have connected the tablet device and the
cradle, the cradle 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.
If you have not turned off your tablet device properly, the controller that controls the power supply may stop due to a failure of the
program. Press and hold the power button for about 10 seconds, or press the reset button to force the tablet device to turn off,
and then turn on the tablet device again [Details] .