What happened is your zoom spring broke. You might be able to get the spring from this place: www.CamerasAndParts.com They pretty much have everything for the Powershots. They can probably install it too if need be, they are excellent for help with the Powershots, that's all they deal with. Good luck with it, that's a great little camera!
I went through this on mine. Get it serviced, once they are serviced right by a technician, the cameras perform beautifully. I think they just don't always come out of production perfectly. The place I would recommend (I've used them twice and they are exceptional for Canons - it's all they service) is: www.CamerasAndParts.com They are very affordable as they have flat rate service packages, their work is superb. Never had a problem with either camera after service, they do some tweaks to make them better than the way they come from production. I would really recommend them if you need help with this lens error problem.