HP 12c Quick Start Manual page 3

If the calculator did not display the "Pr Error" message after resetting, or it did not
correct the error, then remove the batteries for 24 hours.
NOTE:
The batteries must be removed for at least 24 hours to drain the capacitor completely.
After letting the calculator sit without batteries for at least 24 hours, replace with
fresh batteries.
If the calculator continues to malfunction, it may need to be sent in for repair.