The machine firmware is damaged, the first link to the computer to see if the computer can be identified, if not identified, that your machine has become a white apple, the solution is to re-refresh the machine, to download a suitable 3GS version of the offline firmware upgrade package, it is recommended to choose Wei Feng, because the domestic server, stable and support thunder. But be sure to select the correct version number.
Then it's a matter of connecting your phone to your computer, holding down the HOME button and the lock button, releasing the lock button at the tenth second mark, not releasing the HOME button, and then the computer will recognize your iPhone.
When iTunes shows your machine, hold down the shift key and click Restore. You will be asked to select firmware to refresh, find the offline firmware upgrade you downloaded, refresh your machine, and wait for the machine to recover.