DIAGNOSIS The Nest is in a software-imposed safety cooldown to protect your HVAC compressor from short-cycling. This is triggered by too many on/off cycles in a short period.
FIX Wait for the delay to expire. Do not restart the thermostat. Check for these causes: a dirty air filter restricting airflow, a clogged condensate drain line triggering a safety switch, or an outdoor unit blocked by debris. Address these issues to prevent future delays.
IF THAT DIDN'T WORK The delay may be caused by a power interruption. Pull the thermostat display off the base. Wait 30 seconds. Reattach it firmly. This performs a hard reboot and may clear the delay.
HEADS UP Frequent 'delayed' messages indicate a system problem, not a Nest problem. Forcing the system on can damage the compressor. Have an HVAC technician inspect the system if delays occur regularly.