weight

So guys, I went from 115lbs down to 85lbs ... I've been trying to gain weights for the past 3-4 months and nothing has been working, I'm sooooooooo not happy with the way I look. I need to gain weight fast, somehow. Suggestions? I'm used to being heavier and it's bothering me to the fullest extent. I look in the mirror everyday just to say how much I hate how skinny I am.