If you haven't already you should start with the basics. First place to start is to check the transmission fluid with the engine warmed up and running and in park. If the level is ok take a sniff of the fluid on the dipstick. Is it burn't smelling? Also is it dirty or brown? if so you're probably looking at major trans repairs. You should also check the shift linkage on the transmission to be sure it's moving when you move the shifter thru the gears.

