It could be a bad blower motor, bad blower motor switch on the dash, bad blower fan resistor and high-speed relay, blown fuses, bad wiring or grounds, etc...You'd have to use a test light to test for power at the blower motor to see if you are getting power at the motor...if yes, the motor is probably bad or has a bad ground....if not, you'll have to test back through the circuit to see where power is not getting through.
Autozone.com has free access to wiring diagrams for certain models under the Repair section...you just have to sign up for free.