When you can't recall your screen lock:

If you activated a Google Account on the phone, after 5 unsuccessful attempts you'll be prompted to enter a phrase to verify that you're attempting to unlock the phone. Then:

If a Google Account wasn't activated on the phone before setting the lock (or you also forgot your Backup PIN), the only way to regain access to the phone is to perform a Hard Reset which erases all of your user data.

CAUTION

If you perform a Hard Reset, all user apps and user data will be deleted.