Could be low coolant, stuck thermostat, bad heater control head (if climate control)
Ask this question again. Is the question no blowing or blowing cold?
You should check the thermostat and the fuse for the heater.
gas leak or wrong wiring
THE CAUSE for this is that when your cold air is on the petrol is used to create air. and that is how cars create cold air
Needs a recharge
If the unit started it`s probably not the stat.
Compressor not operating.
Apparently when its hot it should be higher than the cold level I read this in my owners manuel .. not very helpful..
Your coolant level is low.
here is what you need to do you need to buy one that goes cold and hot you only bought the one that goes hot
too much anti freeze, bad or no thermostat.
Air in the water system.