I had this same thing happen to me with my old 96 Impala SS, it had the 4L60E and I was driving calmly when all of a sudden I was hitting the gas but the engine would just rev and not pull forward anymore. I had to put the shifter into 2 and then I was able to drive it till I got home. It would only give me 1st gear and SLAM into 2nd gear. I had to have the trans rebuilt and there were a few broken hard bits in the trans. Limp it home, it'll be a long ride in 2nd gear, but it should make it. Or it'll have to be towed, as something broke inside your trans and it definitely will need to be pulled out and rebuilt.