Check the transmission fluid. If there's been a leak and lost too much then it won't get correct operating pressure. Check the colour of the remaining fluid - is it red? Red is good. Just check under where the car has been sitting to see if a regular tranny leak has occurred.

Also check the filter. If it's clogged then that'll cause no or low pressure also. If it's clogged badly, then something inside the 'box has started to disintegrate. Possibly brake bands.

Torque converter is probably not the problem. Can you start it in drive and neutral? If so, then not inhibitor problem either.