Dead Tranny? What to Check? (1981 300D)

I’m wondering if my automatic transmission in my ’81 300D W123 is completely dead or just needs some good diagnostics. I’m asking for help on what to check to conclude what needs to be replaced.
When accelerating from a complete stop I heard a loud bang coming from under me near the transmission. I coasted to the side of the road to find that every gear acted like it was in neutral, except for park which acted like it should. There were no grinding sounds, no slippage or flaring, or any other symptoms leading up to the incident (other than the typical hard shifting when engine was cold), just a load bang and then no torque getting to the drive shaft. I pulled the pan off the tranny and the fluid color looked good, no burnt smell, and only marginal metal flakes in the bottom of the pan, no more than I would have expected for the age of the transmission. There was plenty of fluid and it was changed according to recommendations.

Is the tranny dead or might it hopefully be something less severe? What steps should I take to narrow down the possibilities?

W123