Front windshield vent not working (defrost vent)

I just brought this car, and yesterday my front windshield started fogging up. I tried to turn on the vent to defog the window, and it did not work! Every other vent works in my car, it is just the front windshield, my most important one, that does not work.
The other vents work as they should, they heat up as they should. Even though it takes a while to warm up, and releases cold air when it should.
What can I do, and what is an estimate of how much it will cost me?