As PJD1 suggests, it could be that the key itself has stopped working for some reason. Did you spill something on the keyboard or drop crumbs into it? I have an old laptop where the right cursor key has stopped workng for that sort of reason – I've never bothered to try to fix it, just work around.
If this was a desktop with a separate keyboard I'd suggest turning the keyboard upside down and shaking vigorously. Since this is a laptop, either find out how to remove the keyboard yourself on that particular model (it's usually fairly straightforward), or take it to a repair shop for them to do it.