Easilyconfused

Hi, hope someone can give me some advise. My daughters A1 ran low on coolant some time ago. Since then the cabin heater has failed to produce any heat. The pipes to the heater on the engine bulkhead do not appear to getting warm. I have just replaced the 85 degree thermostat and the problem remains. I feel there may be an airlock issue. I can find no information on bleeding the coolant system. Any advise would be much appreciated.