@ThousandYears - there are lots of resources online that can offer you tips on how to gain weight if you need to. A couple things that might help to search are "weight gain tips" or "weight restoration". In general, eating calorie dense foods, making substitutions (e.g., whole milk for skim milk), and adding calories where you can (e.g., add a little butter to everything, spread some peanut butter on your toast, etc.) can make a big difference.
Take care
