Hi. I'm David, and I'm happy to help you.
Just to be sure: Does the F5 key fail across all functions, not only for the keyboard back light? For instance, if you open a browser and press F5, does the page reload, or does nothing happen?
If the key doesn't respond at all, could you test the keyboard on another computer? And if it's a built‑in laptop keyboard, try connecting an external one to see if F5 works there. While the external keyboard won't control the back light, it should still refresh a browser page.
These checks will help confirm whether the issue is software‑related or a hardware fault in the keyboard.
Let me know the result.
Feel free to ask any questions you may have.