This could potentially be a number of things, such as:
Blown fuse (might affect other systems though)
Heater motor failure
Faulty switch
Wiring loom damage (e.g., from overheating)
Loose connector
Resistor pack (usually still works on setting 4 though)
This is something that comes up quite frequently in the Grande Punto and Evo forums, so it's worth doing a search of those for "heater not working" or similar.