The a/c unit would not turn off

I live in a rather large house. In fact, some might even go as far as to call it a castle. My lake home is located on the coast. It’s on the side of a rather large cliff overlooking a big lake. It is made of stone & it has a tall exterior that would be pretty much impossible to climb. I suppose I just described a castle. Anyway, I live on the coast and it’s in a pretty temperate part of the country where it gets both hot in the summer and really cold in the winter. A lot of things about this home are really super old and the HVAC system is one of those things. In fact, since moving in about a nine years ago, it’s been a big struggle trying to get the HVAC plan to work correctly. We’ve replaced the air ducts, changed out lots of parts in the gas furnace & the air conditioner, & still yet new things seem to keep finding ways to get screwed up! The latest problem involved the thermostat. At some point, the thermostat somehow got stuck with the air conditioner in the on position. This caused it to get way too chilly in my house. I was out of town on company business at the time that this happened, so when I got back to the house, it was pretty much like walking right into an ice castle. I called the HVAC corporation right away. They sent someone out to my house in just a few minutes. Apparently, we had something stuck in the thermostat & whatever it was was causing the system to not shut off at the standard temperature. What a headache.