how canfix my car grand voyager 2.8 auto diesel 05 plate?

I was on M11 doing 70mph. I excelorated to over take a car. I lost engine power black smoke started coming out. I had no engine light or warning light. Engine stayed on would not drive above 20mph. The revs will not go above 2000. There is no noise coming from my engine. Please can you tell me what you think it is so I dont get ripped off.