How do i restart my canon powershot a3100 is camera if it wont turn on?

Make sure the battery is installed correctly.Try opening, then closing the cover to the memory card and battery compartment while holding pressure against it until you hear it click closed. Then try to power it up. The camera will not power up if the cover is not properly closed.

your camera use a USB Cable 3' A To Mini B - (5 Pin). I recommend
1.- connect to another USB port.
2.- connect to another PC
3.- try another USB cable

You can buy in http://www.amazon.com/Canon-PowerShot-A3100-Digital-Camera/dp/B003ZRQQ26

Lens errors are fairly common. Usually it's sand or grit interfering with the lens extension mechanism. Or the camera's been dropped with the lens extended. Or the camera has been powered on, but the lens had been blocked preventing its extension. Or the battery ran down with the lens extended ... Here are some troubleshooting steps that you can try that may (or may not) correct it. They only seem to work for less than 40% of the lens errors, but if the camera is out of warranty (or repair cost approaches that of the camera), they're worth that try. Some of the later steps do involve some risk to the camera, so carefully weigh your options before deciding to conduct them:

It always comes because there is something loose inside the lens, such as a screw. These articles maybe helpful for you: Lens error, E18 error and turn the power off and on again Good luck

The lens is jammed. Usually dust or grit gets in the mechanism and causes it to stick. Could try blowing down the side of the lens but it will probably need taking apart and cleaning. It is not an easy job to do but it might be worth trying as it would probably cost too much taking to a repair shop.

Lens errors are fairly common. Usually it's sand or grit interfering with the lens extension mechanism. Or the camera's been dropped with the lens extended. Or the camera has been powered on, but the lens had been blocked preventing its extension. Or the battery ran down with the lens extended ... Unfortunately, many cameras that fall prey to this can only be corrected by professional repair. But, here are some things that you can do that may correct it. They only seem to work for less than 40% of the lens errors, but if the camera is out of warranty, they're worth a try.

A lens error on these can happen after a drop as you said had happened. It's best to have it go into a service center, they can service the camera and replace the broken part and have you your camera working perfectly again. The one that many people go with for the Canon Powershots is: www.camerasandparts.com They are not much at all and they do excellent work. They're about half the price of what Canon wants and they include all of the service, parts and everything else. If you need help with it, they'd be the place. Good luck with the problem.

Manually rotating sensor rotor inside camera and cleaning gear fixed the Canon "lens error" for me.  
I placed instructions with photo's here: http://www.avh.dds.nl/Fix-Canon-SD600-Lens-Error/

I have the same problem, but i didnt drop mind. The lens where out when i put on da pack and now it won't start.