my ford fiesta won't start?

I know my battery is fine and the car was running perfectly ok. turned off the engine and didn't drive it for a while. Have now gone to start the car and there is just a loud clicking noise - the engine will not turn over at all. all the lights on the instrument panel come on ok. I would appreciate any help anyone could give me! Thank you. My car is a 1998 Ford Fiesta with a 1.25 Zetec engine.