You go to the doctor and find why you have lost your appetite. If not tied to depression or an emotionally draining event, loss of appetite is NEVER a good sign.
If indeed it is due to depression etc and you have found help or have recovered, plain Sustagen or generic equivalent is used clinically to help post-op patients recover lost mass.
I had a friend who had a collapsed lung and contracted pneumonia in hospital - he was skeletal. They gave him a big tub of Sustagen to go home with.
Other than that, it is likely you are anaemic and vitamin deficient all-round. Iron supplementation is crucial. Heme iron is best (from animal sources), but any iron supplement will do.
I suggest you eat eggs as well as milk. Eggs especially are well-tolerated and packed absolutely choc-full of vitamins and mineral sessential for your recovery. You can cram in a lot of goodies for not much food bulk, which makes it superior at the moment for you, to trying to eat huge bowls of salad etc. Milk is also god for those who struggle to eat.