There are lot of reason for weight loss which are depression, Hyperthyroidism, Diabetes, liver diseases. But most probably not eat the healthy diet in the right time is the main reason for weight loss.
Get up in the early morning and drink more water. Milk and milk products are rich in calcium,protein, Phosphorus, Potassium and Magnesium which builds body. Proteins are the body builder so take foods which are rich protein. So take cereals, pulses( if sprouted it is so good). Take this as evening time snack. Take more fruits especially bananas, kismis, fig, dates.
Cashews, almonds are good which gives good fat. Instead of oil use ghee as much as possible. Gingelly and gingelly oil good for health. If your body is not you should take fenugreek to control your body temperature.
To put on weight compulsory avoid the junk foods which immediately gain some weight without any benefit. To control the intestinal worms eat neem(tender) leaves in the morning time.
Soak the Amla with honey to encourage the Haemoglobin or take Nellikai Lehiyam. The other option is take thettrankottai lehiyam.
Apart from this do your work actively which makes good appetite and this enhance the absorption of nutrients. Do it regularly which gives you positive result as a healthy weight gain.