If you have forgotten the screen unlock pattern, you have up to five attempts to try and unlock your phone. If this fails, you can unlock your phone by entering your Google Account user name and password.

On the unlock screen, attempt to enter your unlock pattern five times.
When prompted to wait for 30 seconds, tap OK.
If the display goes off, press the POWER button to turn the display back on, and then press the bar on the lock screen and slide your finger down to unlock the screen.
Tap the Forgot Pattern button in the lower right corner when it appears.
Enter your Google Account name and password, and then tap Sign in.
You are then prompted to create a new screen unlock pattern. If you no longer want to use a screen unlock pattern to secure your phone, tap Cancel.

If you cannot turn on your phone or access the phone settings, you can still perform a factory reset by using the buttons on the phone.

With the phone turned off, press and hold the VOLUME DOWN button, and then briefly press the POWER button.
Wait for the screen with the three Android images to appear, and then release the VOLUME DOWN button..
Press VOLUME DOWN to select CLEAR STORAGE, and then press POWER.
Press VOLUME UP to start the factory reset.