I have a 200r4 in an 89 Turbo Trans Am. I was going to drive my car to work Monday morning and when I got out of my subdivision onto the state highway the car would not shift and I was stuck in 1st gear. I checked the dipstick for fluid and it was about 1/2 quart low and didn't smell burnt. The transmission was rebuilt 20,000 miles ago and the car has 77,000 miles total. What could be the problem?