Nokia 3510i hard reset
Restore factory settings
To reset some of the menu settings to their original values, press Menu, select Settings and Restore factory settings. Key in the security code and press OK. For example, the names and phone numbers saved in the phone book are not deleted.
If it is not working try this code:
You can format your device by typing *#7370# in standby mode. Before the device is reset, you need to provide the lock code. The default lock code is 12345.
Formatting the device will permanently erase all user data and settings as well as add-on applications and operator specific settings.