While my body's preferred mode is to gain weight, I went through a period of time (about
3 years) where I couldn't gain weight no matter what I did. I ended up 35 lbs underweight, but I had completely lost my appetite and couldn't eat much at a time, so at least I had something to work with - eating more, and I did, slowly.
For those that are losing weight, add up your calories you get in a day and make sure you get enough. There are some good food journals out there - find one that makes sense to you. I liked the online version, as I was having to send it in to my doc monthly (
/www.myfitnesspal.com/).
Keep trying to eat, even just a couple of bites, many times a day.
Look for those snack that you can add more calories to by adding an avocado, nut butters, protein powder or anything that will increase your calories in a healthy manner. I used to add protein powder to my nut butter - sort of odd at first, but I got to where I nearly looked forward to it because it went down easily.