How Can I Gain Weight?

hello, i am 38 years old, during years 2008 I had been sick due to my stomach pain on the bed in nagpur medical college near about 4 month, by paining all taste had been made from pathologist and report is normal but my body thin on stick as like typhoid patient after he founded that no blood in my body by that time he told that my leaver is bad position so he has changed my blood minimum 4 bottles , by trying his best I am okay but my legs is gone that is I am not properly paddles walk wherever i gone I had need a stick 3 month I had feel that ghost is walking. By that time I visited to (ngp) medicals Doctor, Private Doctor and Shri Ramdev Baba's Doctor but in vain he said to me that you could not properly walk in your life now, he gave me Tablet , Masaj Oil also to make healthy by ramdev baba doctor & private doctor . After that I'm daily 3 or 4 km paddling on the way wherever i were gone by my intensity and I had go to work on my service today i am stand on my own leg. by that time I'm normal, healthy & fat, my weight near 52 kg.
4 years has been completed today my weight near 40 kg. so pls. give me advice what I do for my weight. ( medical has been given what's treatment to me )