The answer to 'why is my HVAC system blowing cold air' is that there are several common causes, including a refrigerant leak, a problem with the thermostat, or an issue with the compressor or other components. Symptoms may include the air coming out of the vents feeling much colder than normal, the system running constantly, or the air not feeling as cool as it should.
If the issue is a refrigerant leak, you'll likely need to call an HVAC professional to locate and repair the leak, then recharge the system with the proper refrigerant level. A thermostat issue may be something you can troubleshoot yourself, like checking the batteries or settings. But if the problem is with the compressor or other major components, it's best to have a technician diagnose and repair the issue.