Hope you are doing good :)
There needs to be a valid reason as to why your weight has come down drastically.Stress and an improper diet can be two reasons why you have been losing drastically.Even after you eat good and have a good appetite you might not be eating enough nutritious food.
Consult a Dr. once if you haven't
Tips which can help you in gaining weight - 1)Eat small and frequent meals , if you are eating 3 meals in a day break them down to 6 and try and have meals every 3-4 hours. 2)Even though you might feel that eating a burger,pizza with some coke and mountain dew would help you in gaining weight ,Beware!!.These foods can make you fat in the long run.Eat good and healthy food by that I mean good amounts of protein,healthy carbs and good fats.You need to build muscle and not a storehouse of fats. 3)Eating protein rich foods i.e foods like milk,paneer,chicken,eggs,fish and cheese becomes important because without enough protein in the diet the entire food would end up turning into carbs and ultimately fats. 4)Eat calorie dense foods like nuts(almond,walnut,cashew,pecan nuts,peanuts,dried fruits) and good amounts of fruits and veggies. 5)If you have to eat carbs then make sure you choose complex carbs like oatmeal,brown rice,quinoa and barley.Do not have too much of simple carbs(candies,doughnuts,pastries,cakes),remember these shall make you plum and not healthy. 6)Exercise!!Yes weight training can be your best friend if you have to put on weight and build some muscles.Resort to compound exercises like squats,bench presses,chin ups,pull ups and dips. 7)Sleep well,by that I mean sleep for at least 8 hours and while you are sleeping the body is building up some muscles. 8)Do not skip meals as if you have to gain weight you need to eat frequently.