There is no password for a group in Windows...Administrators is a local group.
Just log into the PC as a local admin and then run the "Start LSA" (which really just opens a web page to the UI...LSA starts in the background) and enter the same username and password you use to log into the computer.
Note that if you don't have a local login (i.e., use a Microsoft online account) or your password is blank, you probably can't authenticate to LSA. In that case, make sure the local Administrator account is active, set a password for it, and use that.