2003 Honda Accord Defrost

There is a blend door motor under the dash attached to the "heater/air conditioning box." This little motor will open or close a door depending on the setting you chose on the climate control.
If this blend door motor isn't working or the door is sticking, you will not get heat on that side.

You'll have to look under the dash, locate the blend motor, operate the climate control and watch the motor to see if it moves when you chose defrost. (Compare it to the movement of the one on the other side, if necessary).
If the motor is not moving, check to see if it is getting power. If so, replace the motor.
While the motor is removed, open and close the blend door by hand to be sure that it is not stuck. Turning it one way will provide heat, the other way will block heat. If all that is good, then you know it could only be the motor causing the trouble.