Not kowing how old your system is or what it is, its a stab in the dark. Most systems will have a blower speed for heating and one for cooling and with some a speed for blower only. The control board along with the call from the thermostat will decide what speed is needed. What im getting at is you may have lost the output on the control board for cooling or just lost a blower call from the thermostat. Not knowing if its gas, resistance heating or heatpump makes it hard to say they all have a different blower control.