doesn't go on any forward gears

Cadilac Catera 1998, 61,000 miles. Car doesn't go on any forward gears, but works fine on reverse. recently had a transmission changed. The transmission exchange was because the oil got mixed up with the water.
before that the car would go slow/numb for no reason. Which would fix itself by turning off the engine and starting it again What could cause the problem? thanks you!