What you have posted is basically "It broke. Why?"
If you think you would not be able to answer such a vague question about ANY topic, you are correct. Please write the numbers you are inputting, what the problem is asking, and what the answer you get vs the expected one. Else nobody can possibly help you.
Although I work for the HP calculator group as a head developer of the HP Prime, the views and opinions I post here are my own.
Please tell us the exact details of the problem you're trying to solve, the exact keystrokes you're using, the result you're expecting, and the different result that you're getting. Without that information, there's no way we can guess what you're doing wrong.
EDIT: Oops, while I was writing the above, Tim posted his reply. Unfortunately I can't delete this redundant posting now.