You can reset the password ie security code with the help of master security code.
The master security code can be generated from the websites by providing the imei no.
http://www.unlockme.co.uk/securitynew.html

You have not mentioned the model .If your phone doe snot support master security code try hard or soft reset
http://en.kioskea.net/faq/470-how-to-reset-your-nokia