Reset a Nokia phone
Soft Reset
Soft Reset will clear pending errors and bring your mobile to an initial state by restoring Factory setting. To operate this function on your mobile, the shortcut is: *#7380#
This function apparently works on almost all nokia mobile phones.
Hard Reset
It may happen that despite you have restored your mobile to factory setting, the error still exist. In this case, it is recommended that you opt for Hard reset
Enter the code: *#7370# and press Yes to confirm initialization.
Important Notice: It is recommended that you save all important information on your phone (e.g: memo, contacts etc)
To make a Hard Reset on a Nokia phone that is not willing to turn on, just press simultaneously these 3 buttons:
on/off + * + 3
Before making a Hard reset better backup all your important information: contacts , memo…etc
Reset a Nokia 5800
Press simultaneously the 4 buttons:
Green button
Red button
Camera button
On/off button
If you perform properly the above manipulation the phone restarts and you must enter the time, date …
Reset a Nokia N97
If your Nokia n97 freezes at startup, and the nokia logo is displayed,
You can reset your N97 while pressing the 4 buttons describe above:
Power button
Space key
Delete key (left arrow logo)
Caps /shift button (up arrow logo)
Source : http://en.kioskea.net/faq/470-how-to-reset-your-nokia