In order to hard reset a Nokia 3100, you should perform the following steps.
Remember! Hard reset will delete all the data on your device; pictures, contacts, messages, apps and more.
Nokia 3100 device will be brought back to the default configuration of a brand new device.
1. First, press and hold the Power button to turn on the phone.
2. Press the Call button and tap in the "secret" code *#7370#, confirm.
3. Now tap in the lock code. Default lock code is 12345. Your phone will now reset. Voila!