Hi, Experts

- I hope someone might have some ideas for me on this one. I've searched across the internets and haven't seen anything quite right.
I've got a Canon Rebel with a Sigma 28-135mm lens. A couple of weeks ago, taking photos during a school play, I was "playing around" on the menu and such - and I messed something up. At least I think I did, but now I'm not sure.
The problem is that the autofocus setting on my lens will not allow the camera to take the shot - inside the viewfinder, the focus light circle is flashing after focusing, and the documentation says this means the focus isn't right. At this time, the red AF lights don't show at all in any setting, and the only way to take a photo is full manual. Not a problem, but for Christmas I need to have something a little quicker and less painless.
Any ideas? I've run out of sources, next step is a new camera.
thanks!