Dr. Squat - Dr. Fred Hatfield

I just noticed -- guess I don't spend much time looking in the mirror -- that there is a significant muscular imbalance in my vastus lateralis (front outside quads).
My left leg is fine. The vastus lateralis flexes as one large muscle from just above the knee all the way up to my left hip.
My right leg, however, seems to be suddenly missing the lower half of its vastus lateralis. When I flex the right leg, there is no evidence of any muscle at all in that five-inch area above the right knee. In fact, it appears there is a sunken area there which I don't recall ever noticing before.
I'm 49 and wear sweats or wind pants when I work out. I say that because I'm not honestly sure if this imbalance is something that has been in place for a long time or if it is due to a recent injury or stress that could have somehow atrophied the muscle?
Anyway, I'm sure there was a time when that muscle was identical to its counterpart on my left leg.
No. 1) What hapened to my missing/or/shifting vastus lateralis?
No. 2) Is this common?
No. 3) What exercises should I do to rebuild some muscle in that outside quad area above the right kneecap?