Single listing sales fell off?

There’s this one ASIN I’ve been selling for the past year I would sell roughly 3-5 times a day. The past couple months the sales have completely fallen off, i’m selling maybe one a week now. The listing has no sales rank (never has) that I can track, and I’m still winning 95%+ of the buy box. I’m not sure what happened, and whatever happened it did it overnight. My only guess would be someone stopped paying for advertisement? Can I advertise a listing that’s not mine? Relatively new here. Thanks.