Hi the part you need is called the brake assembly or eddy brake assembly. What happens is the magnets inside the brake have a little cable attached to them and the cable can snap, this in turn lets the magnets go all the way out, causing hard resistance. (its like if you drive around with your ebrake stuck on in your car) You can order a new brake from www.fitnessrepairparts.com or www.treadmilldoctor.com
It looks like there are 3 models of your machine. You will need your exact model # to order parts. Its located on a sticker on the frame near the floor. You may need to tip the machine on its side, sometimes they are located on the bottom of the frame facing the ground.