I don't know if this is a known "issue", but i've been in a strugle to get back main admin login after a power surge in my localhost pc, so if anyone has had this same issue for any know reason, the problem is in the session database table.
You must delete the session of the admin (of course you need to edit directly the database....), because without deleting it, you can't login, even if you have the correct password! And there's no warning message, nothing....
If this is already know, please excuse me....