The answer to 'thermostat blowing cold air' is that the thermostat is likely set to 'cool' mode and the temperature setting is lower than the current room temperature, causing the HVAC system to blow cold air. This can happen if the thermostat was accidentally bumped or if the temperature settings were changed.
Some other common causes include a faulty thermostat, a problem with the HVAC system itself, or a power issue. If the thermostat is blowing cold air but the home isn't cooling down, it could indicate a more serious problem that requires professional diagnosis and repair.