This is most likely going to be due to the Heater Blower Resistor having failed.
You can locate it in the passenger footwell, next to the blower motor. There is a brown and white block connector plugged into it. Remove the connector, then remove the 2 phillips screws holding the resistor in place, then remove the resistor itself by pulling it downwards.
You will probably find that the fusable link has failed or maybe corroded.
You can get a replacement resistor from ebay / local scrappy / nissan dealer, etc, or if you can solder, you can bridge the gap left by the broken link, like I did today, then replace the unit in the car - job done.
Rob Ellis
November 2008