Ford Fiestas, those little cars with excellent gas mileage, have some known issues with their clutch assembly. Problems with the transmission clutch can impede drivability, but there's no reason to panic. Knowing what they are and what to look for can prevent major problems down the road.
While shifting gears and engaging the clutch, the Fiesta is known to rev the engine even though the accelerator is not depressed. A common fix for this is adjusting the throttle position sensor.
Over time it may become increasingly difficult to engage a gear while the clutch is depressed, almost stalling the Fiesta. To fix this, have your mechanic (or yourself) check and adjust the slave cylinder and adjust the clutch release level.
The fitting clip on the clutch assembly in the Fiesta is known to pop off if not installed accurately, causing the transmission to simply quit. A zip-tie can work in a pinch, but a fitting clip or star washer can be installed on the pedal to hold the master cylinder in place.
If you are unfamiliar with the mechanical aspects of your Fiesta, consult a professional mechanic for repairs.