400sel "dead in the water"
 

I have a 1993 400sel with 149000 miles.She had a good service record and spent most of her life as a limo.
two days ago the car without warning lost power going up a hill to my house.When i stopped to open the gate she idled for a while and then cut out.The car would not crank again.I initially thought of the starter motor but a friend of mine came over and turned the car over manually.he then hooked up a strong battery and it turned over but would not fire.there was also a slight knock from the engine on the drivers side when it was turning over.
He told me that the chain may have hopped off the cog and dropped a piston??
I got no warning prior to this that the tensioners were failing and with a good service history a)how did this happen and b)where should I go from here?
The cost could run into thousands.
Can anyone give me any advice?
Thanks:(

You checked obvious things, like coolant level and color, as well as oil level and color?

Can you hear the fuel pump run? Could it be time for a new fuel filter?

Did you check all of your fuses (up close and personal, checking for hairline cracks)?

Caps, rotors, plugs and wires are up to par?

Thanks for the reply Larry.
Yes all these are fine.
Since I posted a chap had the same thing happen to him.
He replaced the rails tensioner and chain and off he went.
I am concerned about the knocking when the car was being turned over.
any suggestions.

A compression check would reveal any damaged valves. Check the one you think is suspect first. Or, find someone that can stick a camera inside the combustion chamber and have a look-see.
If you bent a valve, you'll have to take off the head - and that's where things start getting more expensive than they already are. Being able to just replace the rails and tensioner, and driving off, is ...well....it's damned lucky, I think.