Users cannot login. The username field is highlighted red, but there are no error messages. There is no password reset link
Perhaps User 1 (if that's the admin login you're using) is blocked somehow. There could be a module, or custom code somewhere that is blocking this account's login.

I'd try looking through the list of contrib (contributed/non-core) modules for any who's name looks like it might be responsible for limiting login access.

Also, look for any custom modules and look through their code for any hints that it may be limiting logins also. 

If you find a suspect then try disabling the module to see if this resolves the issue.

Another place to look for possible culprit code is in the active theme's template.php file (usually located in /sites/all/themes/THEME_NAME/template.php)