why does my heating not work?

i've got a ford fiesta zetec 1.25 (2000) and recently had aproblem with the heating, to start with the heater would only blow out hot air no matter where i positioned the temperature gauge, and recently i have had it go from one extreme to the other, it is now always stuck on cold, and this being end of november isn't ideal and making it rediculous to drive in winter, any help would be very much appreciated. Thanks.