By this I mean the vehicle starts but won't accelerate properly, if you think the engine is misfiring see section on misfire.
Engine rev’s but car doesn't accelerate would normally be the clutch slipping, if you lift the bonnet after driving you would normally get a strange burning smell. See section on Clutch and Gearbox
Most diesels are turbocharged, these can blow the turbocharger hoses off, and these hoses seem to get more complicated routes with every new model and can be very hard to see if they are disconnected. If you think a hose may have come off (you may have heard a pop when accelerating) the easiest way to test is to go to the air filter and block the intake, this should stall the engine or suck the hoses flat, if it does not you know you have a hose disconnected. See picture of VW/Audi hose disconnected.
The other problem you get is with the EGR valve sticking open, if you think this may be the problem check out the section on EGR.