Cannon g9 lense will not retract ... it is stuck ... it says lense error restart camera ... i have taken battery out ... let it sit without battery .... i have resintalled battery and still recieve error message ... How do I restart camera ??

The problem is actually quite common throughout all camera brands. Usually it's sand or grit interfering with the lens extension mechanism.
The two links below give step by step DIY instructions on troubleshooting and attempting to fix this problem. Click on or copy and paste the links into your browser.
http://camerarepair.blogspot.com/2007/12/fixing-lens-error-on-digital-camera.html

http://www.e18error.com/repair.html

Are you removed the battery, reinstalled it again? Wont work? Lesn mechanism stuck or internal failed; the G9 is a precision instrument. Alot users with this problem report to Canon Service (click HERE), I attached one answer and comment:

On the service sheet it said this:
Examination/Request Details:
LENS ERROR MESSAGE. LENS IS EXTENDED. CHECK ALL FUNCTIONS, REPAIR TO GOOD WORKING ORDER.

Service Details:
We have examined the product according to your request, and, it was found that the optical assembly was inoperative there was no display on the LCD. The optical assembly was replaced. Other inspection and cleaning and parts replacements wer carried out.

And the best news was the cost: $0.00. Everything is back to normal and in good working order. So, if you have the same problem that I did, I recommend going to the link I had in my first post and going through the repair process. Overall, it took 10 days to get it fixed and back in my hands.

While it was really annoying that this problem happened, I have to say that Canon was satisfactory in repairing the problem, getting the camera back to me in a decent amount of time and NOT charging me. They were also good at emailing me when they received my camera and when they had shipped it back to me (complete with 2-day delivery thru FedEx and a tracking number).

So if this happens to you, I wish you the same luck that I had!
END.

Also could check this generic procedure: Lens_Error_Support

Hope helps.