If the blower works on the defrost mode only, you could have a disconnected vacuum supply line coming from the engine. If the blower motor doesn't work on any mode or speed you could have a blown fuse, burned out blower motor speed resistor, bad switch or a bad blower motor.
Blower motor resistor is bad. It is a little circuit board with resistors on it that changes voltage to the blower motor. When it goes bad, you get full speed only.
The blower motor speed resistor is burned out. The resistor is located close to the blower motor and is easy to change. Your local auto parts store will have that part for you.
I think you should look at your fuse panels and see if you have a blown fuse . Some vehicles have a fuse for high and low blower speeds. Your problem is either the blower motor switch itself or a defective Blower Motor Resistor Pack. More than likely the Resistor Pack is defective.
Need to know what year. FYI, the relay for the blower motor only operates the high speed function. The lower speeds are operated by means of the blower motor speed resistor.