Is the power on to the furnace?
If the fan is not running, check the breaker, fuse, and/or the switch for the furnace. If the breaker was tripped, leave it off and check the furnace wiring for loose connections. Also, you should look for burn spots on the control board and replace the board if spots are found. After any necessary repairs, the breaker can be reset.
If the breaker or fuse was not tripped, look into the inspection window of the blower compartment on the furnace. You should see a flashing green light. If there is no light (neither red or green) then the problem could be the transformer, thermostat, furnace control board, the blower motor, or it's run capacitor.
If you have a condensate pump, ensure that it's reservoir is not full. On most units there is a float switch that will stop the furnace from running if the reservoir is full. If the reservoir is full, then check the power to the unit and replace the pump as required.
After the blower motor is running, the fan can be placed back in the automatic mode at the thermostat.