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.

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.