my renault megane has no communication ?

my renault megane alize on an s plate went in to the garage for an electrical fault now it wont start and is showing no communication when hooked up to the computer. It used nearly a quarter of a tank of petrol in only 5 miles and the smell of petrol was terrible. the garage said they fixed the electrical problem which was, the windscreen wipers coming on on their own and when i pressed the brake the lights at the back i.e the indicators lit up. He also changed the spark plugs and oil and filter change at the same time. What could be the problem