The answer to 'emergency HVAC service blowing cold air' is that the most common causes are a refrigerant leak, a malfunctioning compressor, or a problem with the thermostat. Symptoms may include the air blowing from the vents feeling colder than normal, the system running constantly, and the compressor making strange noises.
If you're experiencing this issue, the first step is to check the thermostat settings and make sure it's set to 'cool' and the desired temperature is lower than the current room temperature. If that doesn't fix the problem, you'll likely need to call in a professional HVAC technician to diagnose and repair the issue. Attempting to fix a refrigerant leak or compressor problem yourself can be dangerous and may void your warranty, so it's best to leave those repairs to the pros.