01.Press Power button until your cell phone turns on.
2.Then press and hold Green handset + Red handset + Camera key
3.Release holded buttons, if you see nokia logo.
4.Now enter your lock code. * The default lock code is 12345
5. After that your phone will reset and restart.
 6.That all.

Second method:

 

 

02.Tap this secret code: *#7370

05.Success! 

Third method:

 01.First turn on your phone.

02.After that press Menu button and goto Settings -> Phone -> Phone management -> Factory Settings -> Delete data and restore.