First, before calling an HVAC repair company, it's important to recognize that a lot of seemingly HVAC problems aren't actually HVAC problems. Instead, something outside of the HVAC system might be causing an issue. For example, someone may have set a box on top of a vent, blocking airflow.
Maybe someone simply turned off the AC because they're too cold while you're still too warm.
That said, HVAC problems are pretty common. Dust and dirt build-up rank among the biggest culprits. Dust can block vents, and jam fans, make the heat exchanger less effective, and cause many other problems. Excessive dust could lead to other issues, like excess moisture, which in turn could result in mold and other issues.
Outside of airflow, if the air conditioning unit is old or damaged, it may lack refrigerant. While refrigerant will often last years upon years, it will slowly wear out. Fortunately, by refilling the refrigerant it may be possible to improve cooling.
Many other things could impact your HVAC system. Regular cleaning and maintenance will help the entire system last longer. It could also cut down on not just repair bills but also utility bills. A well-maintained HVAC system often runs more efficiently, reducing electricity consumption..