Cannot Access Admin Panel

I’ve successfully logged in to my website by using hidden url, but when I try to access /wp-admin, it always redirects me to 404 page. I’ve tried to clean up .htaccess files temporary as suggested by some site that I googled before. I’ve tried to rename ithemes security directory too. But no methods work.

May I get some advise here? What should I do? And please just take a note, I use w3 total cache too. 13 hours ago, I was still able to access the admin panel but not for now.

https://wordpress.org/plugins/better-wp-security/