Method 1:  

Three Finger Reset for Nokia Phone

1. Switch off your Phone.

2. Hold down the following three keys, Green (call answer button) * (Star) Key, and Number ‘3' key.

3. While holding The Three buttons, press the power button for switch on the phone.

4. Wait for the message ‘Formatting’ to the screen On Your Mobile and release all keys when you see that message.

Method 2: 

Hard Format of Nokia Mobile Phone

1. Switch off your Phone

2. Now Press following key-*#7370#

3. Enter Default lock code is 12345.

4. While holding All of the buttons, press the power button for switch on the phone.

5. Wait for the message ‘Formatting’ to the screen On Your Mobile and release all keys when you see that message.

Method 3: 

Soft Reset of Nokia Mobile Phone

1. Switch off your Nokia phone

2. Now Press following key sequence *#7780#

3. Enter Default lock code is 12345.

4. While holding All of the buttons, press the power button for switch on the phone.

5. Wait for the message ‘Formatting’ to the screen On Your Mobile and release all keys when you see that message.

Please Note: 

The formatting method will erase all of your Phone data. 

So... Please take a backup before the Formatting