Here are a few things that I can think of in order from most likely to least:
- Recent OS, software, or driver changes.
- You may have toggled a function-lock key.
- Your keyboard key is not mechanically working properly.
If you have updated your OS or keyboard software, you may have altered the functionality of your key mappings. You should check your Logitech keyboard software or driver. Roll back any recent OS changes or software changes. Update your keyboard driver or keyboard software.
Look on your keyboard for a Fn or function key, toggle it and test the calculator key again.
Remove your keycap from your keyboard and clean the contacts. I think most Logitech keyboards have rubber dome keys so it should be easy to clean. Here is a video but you can look around for more if this isn't close enough for your model keyboard: https://www.youtube.com/watch?v=4GAcBfuTDYs