One day I went to back out of a parking spot, and my reverse gear would not engage. I went forward a bit, and then had a guy help me push it into the parking lot. I had all of my forward gears. I didn't notice unusual tacking in the low gears, however when I was about 50-60 mph, my tachometer would read 3000. When I let off the accelerator, it went down to 1000 like it was idling. It did not give me any warning or leak, but just stopped working. The car has over 300K miles on it! The best car ever!! I have never serviced the transmission and therefore I am responsible for the failure. What is wrong and how do I fix it?