Suggestion: Try gently blasting compressed air into the gaps around that key. It might dislodge a tiny lint fuzzball that wedged itself in there.
If the problem is being caused by sticky gunk inside the keyboard (e.g. after an accidental soda spill dries up), DO NOT attempt to clean out the gunk by removing the batteries, bringing the calculator into the shower with you, vigorously shampooing it, then blasting hot water long and hard into the keyboard, then rinsing it with isopropyl alcohol, then smacking it against a towel until it seems pretty dry, then setting it in front of an electric fan for a few days, then replacing the batteries. The reason for avoiding that process is not because it doesn't work (it worked perfectly for me once, but everybody I mentioned it to called me an idiot for doing it that way) but because I hate to see anybody being called an idiot. Not to mention that it voids the warranty.
If you can't get the key to work, see Appendix F in your "hp 12c platinum financial calculator user's guide" for "Verifying Proper Operation (Self-Tests)". If the keyboard self-test reveals that the key truly is not functioning, the "Service Information" section of Appendix F includes phone numbers for all the Service Centers. Even if your calculator is no longer under warranty, they might be able to help you.
Disclaimer: I don't work for HP. I'm just another happy HP calculator user.