Check engine light came on and about 20 miles later gear slipped.

I was driving on the freeway and my check engine light came on, at the moment nothing happened but on my way home the engine felt like it lost power, I managed to make it to my exit but as soon as I accelerated onto the street the car wouldnt accelerate. It felt like the gear slipped. I limped my way home with the gear slipping every 20 ft or so. What could cause the car to be in drive and then have the gear slip?