Had problem with A/C air handler running constantly, turned off power to it until I could determine whether it was a relay or the thermostat, determined it was the thermostat, installed new "white-westinghouse" thermostat, tested heat and air conditioning to verify they worked properly, then turned thermostat "off" (didn't need it for a while due to moderate weather). Now, wanted to turn on air conditioning, and nothing happened.

Checked power to air handler (it is on).
Thermostat has display and am able to change conditions on it, but it has no effect.
At the breaker panel, it looks like the breakers are all on.

What do I do next?