Citroen Xsara 1.4 LX ,engine turns but does not start?

I have a 2003 model citroen xsara 1.4 LX.It was running very well.Untill one night I parked the car after usual driving of about 50 miles.The next morning my car did not start and has not done so since.The engine turns but does not start.The battery and starter motor is fine and the car has sufficient fuel in the tank.One week before breaking down I put in 20 pound worth of petrol in this car but the petrol needle stayed on empty.The few days later I put in a further 20 pounds of petrol and the needle went to half tank and then next morning car did not start.Local mechanics have checked this car but they could not find the fauld.Please can somebody point me in the right direction.
Thank You

Miss Ayesha