How can I resume updating the operating system of my cell phone which has stopped suddenly?

You can restart your Blackberry when suddenly its update stops in the middle. For doing this you have to press CTL+ALT+DEL, and it will restart. Again, if you want to check you Blackberry to convince that, your smartphone is working, then you have to type 'TEST' in status screen of it and enter. You will get to see that your smartphone is testing fully. As per as your statement, you told that OS updates suddenly stopped, but it does not mean that your smartphone is stopped working. It is better to think that every OS update, which is half done as it will stop to the response, for the moment, and you have to restart the smartphone immediately. As you will see that when you restart your smartphone by using the secret codes then it will again to start the rest of the OS update automatically. Even if you see that your smartphone don't reboot after applying the secret codes then you have to take it to the service station to flash the firmware and reload the latest firmware. However, it is the last resort if your problem is not solved. Generally, it will automatically start to update the pending OS and will complete without any trouble. You should keep faith on Blackberry OS.