Here are some of the important codes that might come useful to you if you are a nokia user
*#06# – to get your IMEI number
*#0000#- to view software version
*#7780#- to soft reset your phone (does not delete user info such as contacts, sms)
*#7370#- to hard reset your phone (deletes all user info)
*#2820#- bluetooth device address
*#62209526# – MAC address
For complete restoration (Do this if your phone does not boot up)
How to do it:
1) Remove the battery, and put it back after 10 sec or so
2) Press and hold these three keys together -Green dial key, Star key[*], Three key [3](in some phones green button + red button + camera button)
3) switch ON the phone.
4) Do not let go the three keys until you see the WELCOME screen displayed and just let it complete the boot process