Windshield fogging up?

We have lot of snow on the ground. I've used the "defrost" on high heat everytime I get in the truck and it never fogged up until the other day. I had it on "defrost" and then it got really foggy. I turned the heat up even more and it didn't help. I turned it to cool still didn't help. I opened up the windows but it didn't get rid of it either. Could something be wrong with with the heater or anything? Drove the truck again this morning, same results. Any solutions out there please?