Switch from regular tv to Plasma?

I recntly bought a new plasma tv that i set up about a month ago. the picure for dvds is fine but when I switch back to regular tv it is snowy. I did not do much when switching the tvs all i did was put the cable where the other one was and then plug it in. since that it has been blury. do i need to get HD to make it regular and not snowy?