DEFROSTER NOT WORKING

Hi everyone, I am a bit worried. The other night I was eating ice cream, with a friend and as most of us owners know, the window's fog A LOT esp when its cold outside. I live in Michigan and it was probably 20 degrees f, fortunately no precipitation that night, but back to the problem. I turned the defroster on, and it started loud and working (the usual, strong sucking sound is how i'll describe it) and then it just stopped, now its like a hissing noise. Its been a few days, and it still is hissing when I turn it on. Has anybody had this problem, and how did you fix it? Please and Thank You in advance.