Now, if the Trans Fluid is dirty, or you dropped the Pan and saw metal shavings, the Main Pressure Control in the Valve Body could be stuck closed. That could mean other Valve Body issues exist also. I used to rebuild Transmissions, and it's not too hard, but keep track of all the bolts you remove, and the make sure the CHECK BALLS don't get displaced. You can take each Ball out one at a time, and mark each location with a Scribe, or small Screwdriver and put them back in their respective positions afterward. Now, with a Flat Blade Screwdriver move each Valve back and see if they spring back to the closed position again and not sticking in any way. You'll need Carb Cleaner to clean the entire Valve Body. I hope you have some kind of Air Compressor. If they are sticking, keep moving the Valves back and forth until they move freely. You may need another set of Valve Body Gaskets, since they may tear upon removal. TMI, I know, but it may be helpful.