Support

I am a bit unclear on your hosting setup. There are two possibilities, both of which are documented in our troubleshooter.

If this site is hosted in its own account: Admin Tools administrator password protection issues

If this site is an add-on domain to a hosting account and its files are inside a subdirectory in the main site's root folder: I created a .htaccess file on my main site and I can't access my other domains on the same account

As you can read in the second link I pasted:

.htaccess settings are propagated by default to the children directories. Let us give an example. Let's say that you have a main site called www.example.com and its files are inside public_html. Let's also assume that you have another domain called www.foobar.com whose files are stored inside public_html/foobar. What happens when you launch foobar.com is that Apache looks for a .htaccess in the parent directory (public_html), applies its settings, then looks for a .htaccess file inside public_html/foobar and append its settings to the ones it has already read from the previous file. It will then process the combined rules and decide what to do with the request.

The only way to have your setup work is to create the add-on site's root outside the main site's root. Instead of creating a folder inside public_html create one next to it.