First, check for power (12v) at the fan plug. If you have power, the fan motor is shot. (replace it).You can "hotwire"the blower right to the battery to verify motor failure... don't forget the motor housing needs to be properly grounded or power is useless!!
If no power, check the blower motor resistors (located on the firewall side of the heater box... usually looks like an oval flat plate with three or four wires connected to it. When removed, you will see two or three small coils: they should not be broken. Upstream from that you may need to look at the blower relay. Generally by the time you get to that, you have already solved the problem. If not, get a good wiring diagram before going further!