A904 transmission not engaging in any gear

I have a 1972 duster with a 340 bolted to a 904 transmission drove the car to get exhaust work done and drove the car back with no issues parked the car now the transmission has no gears. the fluid is full and no leaks and doesn't seem to smell burnt. need some suggestions on what to check there is no slippage when the car is in gear it seems like the transmission is in neutral all the time