In some cases, when the iPhone freezes and the user attempts a reset by holding down the home and sleep keys simultaneously for several seconds, the iPhone will prompt the user to connect the device to iTunes so that a restore process can take place. However, in unfortunate cases, iTunes delivers the error message "The iPhone could not be restored. An unknown error occurred," prohibiting the restoration and hence proper iPhone operation.
This can sometimes be resolved by removing the iPhone's SIM card, then connecting it to your computer. iTunes may then reload the iPhone software successfully, after which you can re-insert the SIM card and attempt synchronization with iTunes.