I just purchased a 1990 Nissan 300zx. It was working fine the first couple of days I had it but now I've run into a major problem. It's an automatic and when I shift into Reverse, or Drive the car will not go. In fact when I shift into neutral I cannot then move the car; it's as if it is in Park the whole time. What could be the issue and is it something that I could possibly fix on my own, or would I have to take it to a mechanic?