Computer Won't Recognize My Keyboard Key

I have a Windows Vista w1907 desktop. All of a sudden, when I click on the letter "s" of my desktop's keyboard, my computer doesn't recognize that I pressed on that one. However, all of my other keys are fine and doesn't give me any major problems.

I tried to click on the Number Lock, Caps Lock, and Scroll Lock but none of these helped me at all.

I typed this from my iPhone 5 because I can't type without the letter "s". I don't want to use the Keyboard option on my desktop continuously because I am weary of using it too much.

If anyone can help me, I will give Kudos to them. But please give this soon.

Thanks ahead.