The blower is turned on by a thermostat inside the plenum over the FAU. There is sometimes an override switch right on the unit, and there is almost always a blower ON switch setting on the wall thermostat. If the latter two switches are not turning it on, when the thermostat inside the plenum is bad.
If you are mechanically inclined (and a bit of a contortionist!) you can get that thermostat out. It may just need cleaning, or maybe replacement; they are not expensive. Obviously unplug the FAU before going after that thermostat.