Hey there,
You could log in with FTP then go to:
/wp-content/themes/
And delete it, then load your site with:
/wp-admin/themes.php
Or you could check your error_log, your host can help with this if you’re unsure. An option is also to enable WP_DEBUG. To output the WordPress debugging information you can open your wp-config.php file and change:
define('WP_DEBUG', false);
To:
define('WP_DEBUG', true);
You can also have these errors output to a debug.log file located in your /wp-content/ folder, to do this add the following to your wp-config.php file:
define('WP_DEBUG_LOG', true);
Using WP_DEBUG would put the errors on the site, you can hide those and just have them out put the WP_DEBUG_LOG (if set to true) by using this as well:
define('WP_DEBUG_DISPLAY', false);
Finding out the error would help to diagnose the issue and possibly fix it.
You also didn’t mention which theme this is?
Let me know what you find.