mercedes a160 engine failure?

my wife's a160 (2002) has just died as she was driving along. She said there was a hissing sound, rather like air escaping, then the car came to a halt. On trying to restart the car, the starter just sounds like it is spinning, but not engaging the engine. my first thought was a broken cambelt, but have since found out that it does not have one. There were no warning signs beforehand, and it has only done 55,000 miles.
Any suggestions to what may be the cause?