My 24V transfomer died after compared with other thermostats in my home. But I had hard time to locate it on the attic. The AC handler is in the hallway. Can any expert help me with this? I thank you in advance.

The transformer is usually located in the air handler. Check for short circuits that may have caused the original transformer to fail.

You are right. I located the transformer, right outside of my bedroom. Thanks.
It however is the overflow sensor in the pipe (!) cutting off the circuit. This design will easily hide the simple overflow problem. I bet this caused a lot of unnecessary downtime for many families. But I will still keep my 3 yr old Carrier AC. After I opened it up, everything went back to normal.

Without the overflow sensor you would be calling the insurance company and filling a water damage claim. Glad you got it going.