urgently check you have water in your radiator header tank
If you have not Do Not! drive any further.The other possibility is that your thermostat has stuck in open position in wich case you can still drive it but it will not be very economical,and it will need to be replaced..
I had the same a few days ago with my Ford Ka. I took it to the Ford dealer and they diagnosed a new heater valve required. All in all cost me �101. Works perfectly now.
Apparently is a common fault with Fiestas and Ka's.