Loss of All Power


I have a 2008 E350 4matic Mercedes with 84,000 miles, just bought.

I changed the serpentine belt and its associated hardware, and was in the progress of charging the A/C. I turned the engine off and turned it back on again, but then lost all power. The key would not turn.

I checked all the fuses and relays, but still no luck. I played with the key for several days and twice I heard the steering lock disengage. The key would turn part way and I would have some power (radio, internal lights).

When I tried to move the key to the start position, I again lost all power, and the key would not turn. I then bought a new pre-programmed key and experienced no difference.

Can you give me a most probably cause for this, keyless module or steering lock?

[Personal contact info removed]
Thanks for any advice you can offer.

Don Plomondon