It could be a lot of things, and you just have to try to rule them out one by one. It happens a lot though, so you kind of have to develop a feel for it.
If it's a sudden, inexplicable food preference thing, you can try other foods (including irresistible snacks) to see if your cat is just having a problem with the specific food that you are serving. She might just be tired of the food, or it could be smelling wrong. Maybe a bad batch, or maybe it's like when you are served a food you like for the 10th consecutive day, you never want to eat it again after that... or at least for a long time. Cats can sometimes be like that.
Also, you can get a lot of insight just by observing your cat. Does she seem really enthusiastic at meal time, but after sniffing it the enthusiasm goes away?, or after eating some? That would be very different then if she just kind of says "meh" when you bring out a plate of wet food, without even bothering to check it out, or only check it out in a very half-hearted sort of way.
At the same time you are looking at the food angle, also look at other stuff, the #1 thing being activity level. If a reduction of food intake corresponds with a reduction in activity level then that could certainly be a problem that you would need to see the vet about. Also anything like a runny nose, watery eyes, or a change in poop (runny, hard, discolored etc) would be a reason to see the vet.
PS, someone has to explain to me about the "thx" thing. It's always seemed to me like: "I want to be polite and thank people for their time, but nah, I couldn't actually be bothered to type those extra three letters on your behalf, so FU".
Sorry... just a pet peeve, and something I've never understood.