What is wrong with my car? Service light flashing, slowed down to a halt, shakes

What is your question?

A few days ago I took my car to the shop because it halted really badly while driving, felt like we were hit from behind. My mechanic at first thought it was the transmission, then after four tests and research he said it was the transmission sensor and had to order the part to replace it. He said everything was great again and should be running fine for a long time, which it did for two days then last night i broke down on the side of the road. It did something different this time. The service engine soon light started flashing and the car started shaking then came to a halt. I could inch forward but not drive. I was looking up and saw that this might be a common problem for my make and model, but i'm not sure. I had it towed to my shop today but since it's the weekend I won't know anything until he sees it Monday. What's going on?? Please help!