Replace Joomla site with WordPrss

Client has a Joomla site in the root. I created a WordPress site in a sub-folder. I want the WP site to replace the Joomla site. First I tried just pathing to the sub-directory as in changed the URLs in Settings > General, copied htacess and index.php – edited index.php to the new location. The WP site loaded but the theme CSS was not being applied and I could no longer log into wp-admin. So I set everything back and was able to access from the sub-folder again. (I have done this move successfully on other sites.)
Then I changed the URLS in Settings > General and moved all the content up to the root. Now nothing loads – just a white screen for the site and for the admin. Not even an error message. So I put it all back in the sub-folder again.
Any ideas for why I can’t get this site to work from the root?