My wife's Canon SD200 got stuck with an open lens and the E18 error. I opened the case and wiggled things and still nothing. Then, I turned the camera on and started tapping the edge of the lens casing with the magnifying glass I'd been using (need the battery
and memory chip in place and make sure you've transferred pictures to your computer
before trying this). Voila! The lens closed. But, of course, it wouldn't open and still gave the E18 error. Well, one good tap deserved another and Voila! again. It opened. After a few more iterations - and cleaning the lens casing each time, it was back to normal. The trick is the timing of the tapping. Just after turning on the power to the camera start tapping on the lens casing - not hard - just a light tapping. And don't tap on the glass lens itself -- you don't want to really break it.