Its probably just sticky and needs a little clean. Try shaking out any dust and crumbs and use a cotton bud with ideally, some keyboard cleaning solution.
As a temporary measure, using the number pad to the right of the keyboard with the Num Lock on (not the numbers across the top), holding down the alt key and typing 68 will give you D, 100 will give you d.
^^^and if that all fails, go to Amazon and search for a replacement keyboard. I bought one recently for a tenner and then, when I wasn't too sure about how to put it in, paid the guy in a local computer shop another tenner to put it in for me.