Ok, let see if I got this right. Used mower, one spring loose underneath. Now by loose, do you mean it just hanging there by one end of the spring? I am also asumeing that the spring is not busted? To find out where it goes, look for the clean/worn spots. The loose end of the spring will have a mark from where it was attached. Is the attached part of the spring fixed, or is it on a moveable? I am assumeing it's fixed. Look around under the mower for a pully, linkedge, lever, any thing that the spring would put force into. If you see a hole in the bracket, check it agenst the mark on the spring. Things get worn, and they will fit together the way they always have. Also look for clean aeras. Most dirt will come from under. Look for any clean aeras that match the spring.
Mostly you just have to let the mechine "talk" to you. It was together once before, follow the clues, and it will tell you how it goes back. Good luck.