HP 300s Scientific Calculator - Manual (Page 92)

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Note that you should make separate copies of important data
before performing these steps.
(1) Check that calculation expression to make sure that it
does not contain any errors.
(2) Make sure that you are using the correct mode for the type
of calculation you are trying to perform.
(3) If the above steps do not correct your problem, press the
 key. This will cause the calculator to perform a routine
that checks whether calculation functions are operating
correctly. If the calculator discovers any abnormality, it
automatically initializes the calculation mode and clears
memory contents. For details about initialized settings, see
“Initializing the Calculation Mode and Other Calculator Setup.”
(4) Initialize all modes and settings by performing the
following operations:
  (CLR)  (Setup)  (Yes).
Reference
Power Requirements and Battery
Replacement
This calculator is powered by solar with a battery (LR44)
backup.
Replacing the Battery
Dim figures on the display of the calculator indicate that
battery power is low. Continued use of the calculator when the
battery is low can result in improper operation. Replace the
battery as soon as possible when display figures becomes
dim. Even if the calculator is operating normally, replace the
battery at least once every two years,
Important: Removing the battery from the calculator causes
independent memory contents and values assigned to
variables to be cleared.
1) Press
  (OFF)