Diesel engine

I need to know why a diesel engine expels a lot of white smoke and has no power

It's most likely that water is getting into the combustion chamber, inside the cylinders. Most likely, the head gasket is damaged and you'll have to replace it.
In most cases, you'll notice it in the engine oil or the coolant (radiator water). Check the oil and water; one of them might be a little milky.

Bye, good luck, I hope this helps.

It's clearly a water leak into one of the cylinders.
As luchito2010 says, it could be the head gasket.
Check the condition of the coolant, check the level, and see if it's bubbling or blowing. That will give you a more accurate diagnosis.
Good luck.