How to overcome loss in appetite?!

Depends strongly on the reason for the loss of appetite. If you're on any medications that could cause a loss of appetite (some anti-depressants are known to cause this, for example) then you may need to switch your medication. If the cause isn't that simple then a relatively easy trick is to count calories and simply ensure that you are eating the maximum number of calories per day. Use meal replacement drinks or mixes to keep your calorie count high and to stave off excessive weight loss.

Sometimes you may find it difficult to eat an entire meal (meals can sometimes be over 600 calories a piece!) so instead try switch to eating smaller meals of about 200 calories every two hours. These snacks are easier to talk yourself into eating and will keep your overall calorie count high. Just keep maxing those numbers every day and you'll be fine - it's much easier to eat enough daily when it's a daily goal of simple numbers than a sense that you simply aren't eating enough. By making the target one that is measurable it makes the challenge seem more possible.

If you have a smartphone (or iPod), all of this is VERY easily accomplished by using available applications to keep track of whatever it is you're eating and how much it was worth calorie wise.

All the best!

Stop being sad. Do things you enjoy. Stop worrying. The end result of everything is your death. Enjoy.