1994 Oldsmobile Achieva Transmition doesn't engage

Hi, as mentioned i have a 94 Achieva 3.1 litre, front wheel drive automatic. This summer i bought a new vehicle and this one stayed parked for about 5 months, starting it and driving it only a few times. I sold it to a friend of mine, and a week later, he tells me the transmition doesnt engage anymore. I never had any problems with either the transmition or the engin in the 2 years i had the car, it always shifted well, and there never seemed to be any slip in the gear shifting. It had good power, and the oil levels were always where they should be. What do you think could be the problem?? Transmition Oil and oil filter were never changed during the 2 years i had it, but oil was good color and levels were good also. Thanks