"Not registered on network" error in Android

Follow the steps to solve this problem,

Settings > Wireless and Network settings > Mobile networks > Network operators

Select your network service provider from the list.

How to overcome Samasung Galaxy Y S5300 Pocket "Sleep of Death" problem

I recently bought Samsung Galaxy Y S5300 Pocket mobile phone. It showed the sign of "Sleep of Death" a common problem of Android phones. The sign of the disease is that phone is automatically switch off when it not in use. I went to Samsung service centre and described my problem. They re-installed the software. But problem  not solved. But I updated the OS through Wireless Internet connection and the problem solved. Following are the steps I followed:

1. Connect your Samsung Galaxy phone to Internet connection.
2. Open Settings > About Phone > Software Update > Update Check for updates
3. Allow the phone to receive update from Internet. Wait for the process complete and at last phone will automatically reboot.

My phone survived from "Sleep of Death" problem after this process. I don't know how it works in your phone. My Phone based on Android v2.3 (Gingerbread) OS.