My 4yr. old Carrier heat pump shows no power to the thermostat therefore, nothing works. Checked the breakers and they show good. Could there be a circuit breaker on an internal board somewhere?

Inspect your thermostat for a battery, might be a AAA or even one that looks like a quarter. Those quarter shaped batteries are usually 3 volts but look for a battery before trying anything else.

There is probably a low voltage fuse on the circuit board inside the indoor unit. It should look like an automotive-type fuse. Do not increase size.