I have a 328 bmw 2000 reg which cuts out ?

My car was serviced recently, then I took it on a long journey. The day after it started as normal but lost power and died about 5 mins after starting. When I re started it , it made an awful sound like a diesel car. Any ideas as to what is wrong with it? Some people say it could be a burnt out valve. others say it could be the timing belt?