An SA password of MS SQL Server can be reset by using Windows Authentication. If Builtin/Administrator is present in the SQL Server, you can login with an ID which is member of Administrators group and reset SA password in SQL Server. Just follow the steps shown below:
Step 1: Log in to SQL server by using Windows Authentication.
Step 2: In Object Explorer, open Security folder > Logins folder.
Step 3: Right click on SA account and go to Properties.
Step 4: Type a new SQL SA password, and confirm it. Click OK to finish.
But if Basic Authentication is selected during SQL Server installation instead of Windows Authentication, then an SQL user cannot be able to follows the steps shown above. In such situation one should check some third-party SQL Password Recovery tools. I would recommend here SysInfoTools MS SQL Server Password Recovery here to reset a new password for SA login to unlock SQL Server. It allows users to reset password for both logins, individual as well as SA login.