There's two possibilities in your web.config file for the <membership> and <roleManager> sections.
Your "defaultProviders" for <membership> and <roleManager> might either be DbMembershipProvider and DbRoleProvider, respectively. Or they may be SqlMembershipProvider and SqlRoleProvider, respectively. Actually, the names
could slightly differ.
If you're using the first ones (DbMembershipProvider and DbRoleProvider), then the be_Users and be_Roles tables will be used (these are the "BE" ones). If you're using the 2nd ones, these are tied to the standard ASP.NET membership tables
which usually begin with the "aspnet_" prefix.
If you're using the BE ones (the first ones), then check to see what data is in the be_Users and be_Roles tables.
Also, if you make any manual changes to the DB tables, you'll probably need to restart the blog so those changes are detected. You can restart the blog by making any change to the web.config file (add a space, etc).
If you set the password in the be_Users table to a blank password (an empty string), then you can try logging in via the default password of "admin". But again, if you manually change the password in the DB, make sure you restart the blog
as noted above.