Most likely the blower resistor has failed. It is located under the hood in the side of the heater box.
replace the blower motor resistor
If it has only high speed, the blower motor resistor has to be replaced to get all speeds back.
replace the blower motor switch
The blower motor speed resistor is burned out.
Most likely a failed blower resistor.
blower motor resistor is burned out
Check the blower resister
The blower motor resistor pack is bad.
Blower fan inoperative or only works on high speed.
the blower resistor is bypassed on high. a direct path to ground. if high is the only operational speed the resistor is bad.
If you are referring to the blower motor only working on the high speed setting, then the problem is the blower motor Resistor Pack is defective. Replace it. It can be the blower motor speed control switch but not likely.
when a blower motor only works on high, it is usually due to faulty resistor board. Should be located in the duct near the blower. the purpose of this is so the air blown by the blower keeps the resistors cool.