Gaining weight back

So, when I was a soldier I inexplicably gained 30 lbs (13.6kg) at basic training, but now, 3 years later I've lost 50 lbs (22.7 kg). Why can't I gain back any of it? Suggestions to regain any (preferably all) of it?

(muscle mass, not fat)

Weights are the best option for gaining weight, also a proper diet regime. You might want to avoid cardio for a little while just so you can put on some fat for weight training instead of having no fat and your body starts using muscle as energy. Also did you lose any size or was it just weight.

EDIT: Yukito didn't you try to do this at the start of the year?

^
No. You should consider doing weight lifting if you want to put on muscle mass, but you should still run to build up lean muscle and to prevent excess gain in fat. Also taking protein shakes or whey protein depending on your size helps you build up muscle mass(take it if you are going to weight lift).
Edit: I forgot to add that you should consider changing your diet for example eating 4 meals a day with some form of protein, vegetable, and fruit. Also eat before and after you lift.

Indeed I did, but hunting down the thread seems tedious. Things got in the way and now I'm going to retry.