Losing all motion at once isn't common. Not every failure I've listed below has a noise associated with it but it is good practice to check.

If the electrical plug on the trans was removed, you would still have 2nd and reverse so don't look for a electrical problem yet.

Start with, On a level surface,with the car in park and the parking brake released, try to push the car back and forth. It should move a few inches in each direction then stop.

If car moves farther, look at drive axles, rear differential and input flange, drive shaft rubber discs, transmission output flange. If the nut holding the trans or diff flange comes slightly loose, the splines will wear out and strip. This has happened a few times on this list across a few different models of trans.

Next look for signs of fluid leakage, use a dipstick tool to check fluid. ( at this point you are looking for a drastic 2 Qt down situation not a 1/2 Qt.

Jack the front of the car up and support with stands or use drive on ramps. You need to look at the flywheel / flex plate to torque converter bolts, there are 3 sets of 2 bolts ( 6 total ) , make sure these didn't fall out.

Look through the vent holes at the bottom of the bell housing, identify the torque converter, have someone crank the engine and look to see that it turns. If no rotation, the flex plate has cracked. ( make sure you are not just looking at the flex plate / flywheel )

If the torque converter turns, remove both trans cooler lines and put a pan under them. Crank the engine, fluid should come from one of them. ( you need to have the pan under the trans side of the lines, be ready for a fluid spray. Make sure you are on the transmission lines and not an engine oil cooler. ) The fluid on these trans normally gets dark, only be concerned if it smells burnt.

If no fluid comes out and there is fluid in the trans, the torque converter pump drive slots / trans pump drive tangs have sheared or the pressure regulator spring in the valve body has failed.

If you get a fluid stream, the trans has to come out for a look.

The 722.6 is used on many Dodges during MB's ownership of Chrysler ( and a Jaguar or two ) so knowledge can be found at a decent domestic transmission shop. There is no need to go to a MB specific shop.

Please report your findings so it helps others in the future.