You may be unable to unlock your phone's screen after forgetting your PIN or password. 

The screen lock acts as a security feature to prevent unauthorized access to your phone, so there is no way to retrieve a forgotten PIN or password. To unlock your phone, you will need to perform a factory data reset. See How to factory reset a device.

NOTE: A factory data reset will erase all of your personal information from the phone's memory and reset the device to the factory defaults of the current ROM. Any information stored on your storage card and SIM card, however, will be saved.