Nothing stops working out of nowhere. I am quite convinced that something changed on your host's side. Now, a white page or a page with a 500 Internal Server Error is, in fact, either a .htaccess issue to a PHP fatal error in disguise.
First, let's see if it is a .htaccess issue. Try renaming the .htaccess file in your site's administrator directory to htaccess.bak If that solves the problem it's likely that a change in your hosting setup conflicts with the .htaccess generated by Admin Tools' administrator password protection (if you had used that feature).
If that doesn't help, the error you are receiving is in fact a PHP error in disguise. First, check your server's error logs (not the access logs) immediately after visiting the page which throws the error. There should be an exact description of the PHP fatal error which occurred. Sometimes you can find the error messages in files called error_log or error.log inside the site's root and/or administrator directories. If the file is found in both locations, look in the one under the administrator directory. If unsure about the error log location, please consult your host. Most likely the error logs are available in your site's cPanel, Plesk control panel or similar hosting account management facility.
If your host does not give you access to the error logs and you have access to the Joomla! administrator area, please log in to your site's back-end, go to Global Configuration, click on the Server tab and set the Error Reporting to Development (Joomla! 2.x and later). Try visiting the problem page again.
If you still get a blank page, edit your configuration.php file and put the following code right after the final closing curly brace ( this is what a curly brace looks like --> } ) but before the closing PHP tag (it looks like ?> that is a question mark and a greater-than sign):
ini_set( 'display_errors', true );
error_reporting( E_ALL );
Try visiting the problem page again.
If you still get a white page, please remote the two lines from your configuration.php file. Edit the .htaccess file in your site's root. Add the following to the end of the file:
php_flag display_errors On
php_value error_reporting 32767
and retry loading the problem page.
If you still get a white page, remove the two lines from your .htaccess file. Now, create a file called php.ini with the following content:
display_errors=on
error_reporting=E_ALL
and upload it into your site's administrator directory. Retry loading the problem page.
If you still get a white page contact your host. We need access to the error messages.
Please note that if you can not understand what the PHP error message means, just copy and paste it here verbatim so that we can take a look and point you to the right direction.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!