Have you moved, copied, or cloned your WordPress site and now you can’t login to your WordPress admin area? The first thing to do is to double check the login problem punch list WordPress provides at: Login Trouble. If this doesn’t solve your problem, please check to see if your site is using the W3 Total Cache plugin.
The W3 Total Cache plugin writes custom entries in the wp-config.php file that are URL specific. If you have this plugin installed, even if you disable it, the entries still stay in your wp-config.php file. If you fail to update the URL in these entries to your new site URL, you won’t be able to login to the admin of your WordPress site.
Here’s the entry you’re looking for in your wp-config.php file:
You'll either need to remove both W3 Total Cache entries or update your new site URL in the second entry (YOUR-SITE.com).
There are a number of other possible causes as to why you can't login to WordPress's admin, however updating or removing the W3 Total Cache wp-config.php entries aren't listed as a solution by WordPress or most of the other Login Problem tutorials available, even though it's a common problem source. We've run across this problem several times in the past and wanted to pass along the solution. Hope this helps. Best of luck.