If you're signed up for a saver switch through your utility company, they may be temporarily locking you out during peak demand hours. If that's not the case, then try the following steps:
1. Verify that the thermostat is switched to cool mode and has a cooling set point below the room temperature reading.
2. Check the thermostat's display to confirm that there's a call for cooling, usually indicated by the words "cool on" or a snowflake icon. If the "cool on" or the snowflake icon is flashing, the thermostat is in delay mode, which can take up to 5 minutes. This delay is to protect your equipment from short cycling.
3. If the thermostat indicates a call for cool, verify that the furnace door is securely closed and that the power switch on the furnace is set to On.
4. Make sure the circuit breaker switches for the furnace are on.
If the cooling still doesn't come on, but has worked in the past, call a professional HVAC contractor to assist you.
If this is a new installation:
1. Verify that the wiring was matched correctly from the old thermostat to the new thermostat. If there are not separate wires to R and Rc, these terminals must be connected. On some thermostats this connection is made by a jumper wire connection and on other models this connection is made by a slider switch that has 1-wire and 2-wire settings. The installation guide for your thermostat will show which you have.
2. Confirm the thermostat has been correctly configured for your system.