Automatic Transmission Shifting from 1st to 2nd

I have a 2001 Saturn Station Wagon. I recently had to get a new water pump and front brakes. On Saturday night, I was driving a short distance and had problems getting the car to shift from 1st to 2nd. I pushed on the gas pedal but the car would not go over 20 mph. When I drove the car on Sunday afternoon, again a short trip, I did not have the same problem. Any ideas what this could be? I believe the timing belt had to be removed to get to the water pump. Could this be the problem?

Only if they disconnected the automatic transmission fluid lines to do the job.
Check the fluid level in the automatic transmission. If low, add the correct fluid to bring the level to the mark.