I don't know much Japanese but I am learning. I cant seem to find any help on what and why this short sentence means exactly. My pen pal friend asked me げんきだったの? I am guessing this is asking me how I am doing because of the genki datta part, but the NO at the end confuses me. I'm assuming this is grammatically legit, but I'm at a loss as to what it is. Can anyone help?