Major engine probs

Around 2 weeks ago i sent my 306 dturbo into the garage with an oil leak. as the leak was around the cambelt & didnt know the cambelt history i told them to change that at the same time.
The work i ended up having was
new cambelt (said tensioners didnt need changing)
alternator belt
crank oil seal
full service
new sump
2 days after i got it back i decided to check the oil. it didnt even register on the dipstick.
now a week later i was accelerating when it sounded like a belt slipped, it lost power & drove lumpy while throwing out black smoke. while ticking over it sounds as it always does but wont drive.
any ideas?
i will be taking it back to garage but wud prefer to have an idea b4 they start talking crap
thanx for any ideas
By the way can a different sump be fitted? if so can it effect the dipstick reading? ie show no oil when theres oil in