I gained it back simply by being repetitively injured and eventually quitting competitive running. But since I cannot train at the level I used to (D1 college), I'm not happy with my weight right now. Since my weight dropped so fast in high school and stayed low for the 4 or 5 years I ran competitively, my body now hangs onto every bit of fat. It really messed up my metabolism and I still can't figure out how to fix it. I still work out and run when I am not injured, but can't seem to lose weight. The injuries have not decreased since gaining weight because the root cause of them, low bone density, muscle imbalances, amenorrhea, etc., started when I was UNDERweight.
There's nothing wrong with being thin, as most elite runners are, as long as you are healthy. If you are underweight at an unhealthy level (according to a medical expert), my advice is to first figure out why. If it's dietary, talk to a nutritionist, if it's psychological (depression, eating disorder, etc.), talk to a counselor, if it's just training related, talk to your coach about adjusting your mileage. Just do something other than ignore it.