Hi ryan,
Are you talking about the idler arm tension spring? This one?
[image|3068983]
As to why it may have come off, about the only time I've had similar springs come off on major appliances, it was because the hooks at the end had worn through far enough that they eventually broke. If the spring is intact then that's a really good question. Take a picture of your spring that clearly shows the hooks at both ends and we'll see if we can figure out if it's intact or not. Of course, if the spring is still good, then that leaves the places it's mounted to but judging from the pictures I see of that particular spring, it doesn't seem likely those parts could wear enough to fail to hold the spring.
Post a picture and let's see if we can figure out what's going on here.