Solution:
First try to create .htaccess file from WordPress Codex page.
Next, try to reupload the wp-admin and wp-includes folders.
Update:
You mentioned in the comments you get the following error:
cannot create references to/from string offsets in /wp-includes/user.php
It looks to me that you are using PHP 7 on an older WordPress version.
Try using the fix below:
https://core.trac.wordpress.org/attachment/ticket/37071/fix-wp-login-error.patch