Black smoke, no power

Hello. I have a 96 f-550 with the 7.3 litre turbo deisel. Just got it back from the dealership for a new water pump. Now after 350 km trip, i have no power and thick black smoke coming from the exhaust. It won't go over 1500 rpm and can barely get the truck to move. The truck only has 142,000 km on it.