http://www.nlm.nih.gov/medlineplus/e...cle/003121.htm
Here's a link to possible causes of lack of appetite. The link suggest several possible causes and treatments, among which are to be sure to consume high protein nutrient rich drinks and eat several small meals a day, and keep a record of food intake to ensure that you're getting enough.
Most importantly, see a doctor and tell him/her about this. You may have a disorder that requires medical treatment.
Gilda