We were away for 3 days. Our thermostat was set at 66, but when we returned, the house was an unbearable 96 degrees.Please help. A service technician could not fine anything wrong.

There are numerous possibilities but one that's fairly common yet can be difficult to find is either rodent damage or a nail into the thermostat wire. Often it's easier just to pull new wire than try to find the exact point of damage.

Sounds like you need to find a different technician. Try having your technician installing a different thermostat to see if that makes a difference. Most techs have several old ones laying around.