The instructions below explain how to restore the HP 9S calculator to normal operation.
HP 9S Calculator - Resetting the Calculator
If there is an overflow or error indicator on the display of the calculator, clear it by pressing the ON/C key.
If the calculator locks up, and pressing keys has no effect, press M+ and ENG at the same time. This unlocks the calculator and returns all settings to their default values.
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Country: United States