I'm a sickle cell patient and over the past few years i've lost a considerable amount of weight. how can i get it back or gain some at least?

Is it your diet?: Do you take pain medications that may cause nausea or constipation leading to decreased appetite and/or decreased food intake? How about hydroxyurea (which you almost certainly should be on) ? That can sometimes cause nausea. Accutane is also known to cause weight loss. Ask your doctor help dive into medication-related causes of decreased appetite, if you think that is a possibility.