Hi, ftp into your site and remove the following from your .htaccess file. Also rename the plugin folder from far-future-expiry-header to temp-far-future-expiry-header.
# BEGIN Far Future Expiration Plugin
<IfModule mod_expires.c>
ExpiresActive on
<FilesMatch “\.(gif|jpeg|jpg|png)$”>
ExpiresDefault “access plus 5 days”
</FilesMatch>
</IfModule>
# END Far Future Expiration Plugin
Then make sure you clear your browser cache as well. Then try login into your site.
Let me know how you go.
Regards