1966 A904 loses all gears overnight

If a steady stream of fluid pours from the line that comes from the front fitting on the transmission while running, checked at the cooler, you can assume the the pump and converter are OK, the converter hub is not broken. No fluid at all from that same line while running means trans has to come out. Fluid coming from that line means pull the pan and check for the linkage AJ mentioned, but you are only looking for the bent steel comb that has the detent teeth to have a small tab engaged with and moving a cylindrical valve partially exposed from the valve body. If all is good, trans comes our for teardown inspection. Front fitting on trans comes from pump as cooler pressure, rear fitting is returning fluid to trans by lubing the rear support at the back of the trans case.
Edited to remove mention of wire/rod connecting the comb to the manual valve. Torqueflites don't use that. I had a recent 700R4 on my mind. Sorry for mix up.