There are really only four parts involved here, the fuse, the switch, the resistor pack and the motor itself.
Check the fuse, if its OK, then the resistor pack would be next. If the motor works on the highest setting, then the resistor pack is defective, if not, then it's the motor. The switch itself could be bad but that is unlikely.
The blower motor cannot be economically repaired. It is built in such a way that the brushes cannot be replaced. This seems to be the trend these days, make things that can't be repaired so you have to fork over $200+ for a motor instead of $6 for brushes.