More than likely the blower resistor is blown.
I'm not sure of the location on your car, It may be on the firewall near the blower motor.
If it is there are 2-3 screws holding it in. Remove the srews and pull it out. You'll see about 3-4 coils of wire, if you see one or more of the wires broken, there's your problem.
good luck
========= Harryface =========
1991 Pontiac Bonneville LE 3800 V6 ( C ), Black/Slate Grey _~_~_~297,626 miles_~_~_
~_~_~_~_U.S.A._~_~_~_~_~_
~~~The Former Fleet ~~~
89 Cavalier Z 24 convertible
78 Holiday 88 coupe
68 LeSabre convertible
73 Impala sedan