A couple of things to try.
Adjust contrast with on and + button (or on and - button if screen is black).
Next, try a soft reset, press On, F1 and F3 all at the same time.
nothing?
Try a complete reset, Press On, F1, and F4 (or F6, hell try both at this stage).
Finally try a hard reset, take a small paper clip and insert in Reset hole in back of calculator.
Still nothing?
Try changing the little 3 V button cell and retry option 1 above (you may have a dead 3 Volt button cell or it is in backwards, though this would be tough to do as it is a shaped compartment).
Nothing?
Double check your batteries again.
Then it is time to call Hp and complain!