Defrost not Working in my 98 honda civic?!

Everything works fine except when it comes to defrosting my front window. The heater works fine as well as the ac, I don't know if it's normal but when I turn on my ac without pressing any buttons to direct the air it goes straight to my feet( i believe thats normal?). When I press the defrost button, you can hear a humming sound coming from the defroster but no air comes out at all, instead the air goes to the vents on the sides( not the middle vents) and the feet vents mainly. How can I fix this problem folks?

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