Is this a new thing? Anything else going on? Could be due to depression, a stomach bug that you had recently, a lower than usual activity level. Kind of hard to answer your question with no detail available.
Are you taking any new medications? The side effects of some meds can be a decrease in appetite. And some could just make foods seem unappetizing to you? Try drinking one of those supplements like Boost or Ensure or talk to your doc in regards to an appetite stimulant. But by all means... Ya gotta eat!
I just came back from Six Flags at a hot day and I began to feel sick when we were traveling back home and my hands began to feel numb and almost threw up but held it in. And now the air conditioner is partially broken and I don't feel as hungry. Maybe the same condition your going through?