It could be the motor is jammed, or a dead short, unplug the motor and replace the fuse again, don't be tempted to up the rating of the fuse, this can cause fires and destroy your car!
It could have lost the earth to the motor, ie broken wire, unplug motor and check output at the connector that goes on the motor.
I've known one over the years where the owner always ran with no pollen filter, eventually enough crap found it's way in the heater to jam the motor.
tabetha