Stuck in 1st gear

I have a 2001 V6, when driving it feels as if you hit something in the road and at that instant the check engine light comes on. The rpm's go way up telling me that I'm stuck in 1st gear. I can move the shifter over and drive it manually. Any Ideas of what could cause this?

so your saying that the tranny won't shift when it's in drive? Did you check the fluid?

[I checked the fluid, it's fine. The car jerks like you hit something, then it's stuck in gear.