how many miles does the car and/or transmission have on them?

Could be as simple as lack of maintenance and the tranny fluid and filter need changed. But could be that the forward band broke and you need a rebuild. If I had to guess, I would say the latter.

But check the fluid level and condition of the fluid first. Should be to the top of the XXXs and a nice transparent pink/red color. Not brown, dark, black, or smelling burnt. That is not good.