Antipollution system faulty + clicking noise

Hi,

My Citroen C3 1.6HDI 90 refuses to start. On the display it says 'Antipollution system faulty' and the motor just makes clicking noises when I try to start the engine, like a flat battery. It was working fine 5 minutes before.

Could it be a fuse or a faulty sensor? Has anyone else had this problem and can give me a solution before I take it to an expensive garage ?

Thanks!