2004 Nissan Quest--transmission blues?

Last night, while accelerating after stopping for a red light, I heard a big ka-CHUNK! as if I had run over a stick (there was nothing on the road), and immediately felt that although I was giving the engine plenty of gas, it was not accelerating. It did not sound like the engine was straining, nor did the tach show high revs, but the thing moved like molasses. Once we got up to about 40 MPH, it seemed to have more normal response, although the tach showed 2,500 v my usual 2,000.

The problem was consistent throughout the rest of the 25 mile drive–if I had to come to a stop or near stop, getting back up to speed was very, very slow. Also, when going up hills on cruise control, speed would come down about 5 MPH, which it has never done.

I’ve got 125K on my kid limo.

I think I may have lost my first gear. Anything else I should be looking for or any suggestions? Thanks!