Hold [doc], [enter], and [EE] at the same time.

Make sure battery is fully charged.

While still holding down the buttons, press the reset button located on the back of the calculator.

If the calculator doesn't power on automatically, keep holding down the buttons, then press the [on] key (twice in a row if necessary).

Select [0] first to reboot the calculator, if it does not help, press [2] to delete the OS.

Alternatively, remove the battery, hold down the buttons, then connect the calculator to a power source.

After the 'replace battery' warning appears, connect the battery and try the button combinations again.