Im going out on a limb here, but I strongly doubt that your site doing 1 sale per day has caused some hacker to take the time to setup a dns poison attack. Unless perhaps you ticked off a technically advanced competitor. dns poison attacks at any type of global level would be very, very difficult. Try doing a tracert to your site, does it land on YOUR ip? If so it is not a global dns poison attack. Also, if it was a dns poison attack wouldnt your traffic have died to zero when in fact your traffic remained the same but conversions dropped.