Some models of Nokia such as Nokia N gage or Nokia 7650 must have the key sequence of the phones, exactly the way a phone number is dialled. Here is a particular procedure for resetting the latest number of phones. In order to reset, the first step is to turn off the phone. Next, press the talk button and * button simultaneously and hold the '3' key. Another procedure is to wait until an onscreen message appears on the phone. As soon as it appears, the resetting procedure is completed by releasing the three keys.
Using the *#7780# sequence
It will restore .ini files from ROM in order to keep all your data (themes, 3rd party applications, video, photos, and music files)
Using the *#7370# sequence
Completely format phone memory. Default files are cleaned up and rewritten.
Note that data found on memory card will not be altered
Note: When using a symbian60 v1 based phone such as nokia 7650, 3650 or N gage models, these 2 key sequences are to be entered in the same way you dial up a phone number. The phone will ask you to enter a security code (12345 by default).
On the latest S60 phones you might try a Deep Reset, using the following steps
Turn off the phone
Press simultaneously on the green/talk button, 3 and * keys and hold down the three keys
Turn on your phone
Wait for the onscreen format message and then release the three keys
This method might also work for Nokia phones that won't turn on. Note that before performing a reset on Nokia's latest phone (N96) using the *#7370# or the three finger method, you should make a backup as you will automatically delete important data, including the license for pre-loaded contents (music, video, maps, etc.) and pre-installed applications (N-gage).
You should use this method (reset) as a last resort.
To know more please click here
Note
Thanks to skmr for this tip on the forum.