I wish people would quit taking cars to dealers. Especially THESE cars.

1) Never take a Mercedes to a Chrysler dealer. Sure, this is a problem ANY shop SHOULD be able to fix, but still....

Anyway....

Battery and battery connections - check the tightness of the clamps on the battery (yes, I know the dealer should have done this. Maybe they did.) Follow the ground cable from the battery to it's end - is that lug clean and tight?

See - in the old world, if the battery became disconnected, the alternator would most likely run the car. But with these cars (and not just the Crossfire), the absence of the battery in the circuit causes the electronics to get confused/pissy/etc. The engine then dies. Now, since the battery is disconnected, the car won't start. I would suspect that it is a 90% chance your battery "became disconnnected".

Of course, you said "all power was lost". If this is true, it almost has to be the battery / battery cables / etc. It COULD be simply a bad battery - and it may test OK. IF the problem is a bad weld/connection from one of the posts to the plate at that end, it would be like a battery cable coming loose.

PERHAPS you have an open ground in that little black box ahead of the battery. Or, perhaps you have a bad connection in the ECM/PTCM/RCB box. But I REALLY thing it's more fundamental than that.

Here's a test (and this happened to me about 25 years ago): With the car not running, turn the lights on. Now, wiggle both battery terminals AS HARD AS YOU CAN. First, they should not move. SECOND, if the lights flicker, look for loose battery cable clamps or corrosion under the clamps, between the posts and clamps. If you find none, but the lights flicker, you have a bad connection/weld from the post to the plate in the battery... replace the battery.