If you are having trouble logging into your admin panel or other password-protected page or site, you may be encountering a saved login cookie with a different set of credentials. You may have to delete these cookies to be able to login with new credentials.
Please follow the steps below to find and delete these cookies, then you should be able to login fresh with your new credentials. Of course your website domain will be different from the one shown in the images.
These instructions are for Chrome and Firebox, but each browser works roughly the same way. You don't normally want to clear ALL of your cookies, only those specific to the website you are trying to log into.