In case you forget your lock pattern, here's what you should do:
1. Continue to enter the unlock pattern five times until you are prompted with forgot password/pattern at the bottom right side of the screen.
2. Tap on the Forgot Password option and you would be prompted to sign in with the existing Google or Gmail account information. You need to provide the ID when you first register or activated your phone. If you have that, it would be easy to get in for you and then change the unlock pattern.
3. If you don’t know the Gmail ID or the password is wrong then there is only one solution to hard reset your handset. The data present on the added SD card will not be wiped out but you would lose your personal data or information or contacts stored on your phone memory.
4. For procedure in doing a hard reset on your device, click here. source