It is not possible on the information given to conclude the thermostat is stuck.
Does the temperature guage go up to the level it usually does? If not then it is probably the thermostat. If it does go to its usual level, then it could be a valve in the heater circuit that has failed (but that depends on the age of the vehicle, older ones didnt have a valve but newer ones did: I do not know at what year they had them in Fiestas).
It could also be an air lock, some cars get these and have bleed valves in some hoses, some dont: I have not seen any bleed valves on any of the several Fiestas I have worked on.