More than likely the fan limit switch is your culprit, it's designed to operate the blower fan after the combustion chamber reaches a preset temp and keep it operating after the room thermostat is satisfied and the burners shutoff... The blower will 'cool off' the combustion chamber to a second preset temp on the limit switch... Certain types of this limit switch have a button on the front that will allow you to manually operate the blower motor continuously.. Locate this switch and push or pull the button if one exists and the blower should come on... Turn off the power if you're unfamiliar with this control and remove the cover of the limit switch to find the part number to help with finding a replacement...