Hewlett Packard 42S

[dramsey]

At this point you're ready to go, using the calculator for whatever you want.

The 42S has "continuous memory"-- that is, it retains any information (programs, whatever's on the stack, etc.) when it's turned off-- AS LONG AS THE BATTERY IS GOOD.

In your case, the battery had likely died years ago, and although the calculator can hold enough charge to retain its memory for a minute or so while you change batteries, it can't do it for years! So when you put in new batteries and got it working, it displayed the message you saw, notifying you that it had lost whatever information it had.