when i turn on defroster nothing happens

When you say that nothing happens> what exactly occurs (or doesn't)? Does any air blow out forceably at the windshield, or does it blow but only cold? What about when you run the heater- do you have any hot air, does it blow out on all the selected speeds? Depending on your further description- there are so many varibles to the situation as you explained it. I'd like to help you but I need more information.