90 Silverado tranny not working

Just bought a 90 Chevy 4x4 Silverado,305 engine,automatic O/D tranny.Truck is in good cond.,but while driving it the tranny stopped working completely.Nothing,...no drive,reverse,O/D,,etc...changed fluid and filter also changed govener?..still nothing...any quirks with these trannies that i may try to get some movement.i heard that these things were bullet-proof!!!..any sensors etc. that I should check. I have a spare tranny but don,t want to change it if the old one is OK..Thanx for any help.
btw..this is my first time on this site.Just found it tonite..
Harey Carey