There are a lot of third-party causes that could break a WordPress site after updating WordPress, themes, and plugins. Please do the following troubleshooting steps to weed out the common causes of update issues.
Ensure everything is up to date according to our version compatibility list at https://theme.co/apex/forum/t/troubleshooting-version-compatibility/195. Please follow the best practices when updating your theme and plugins. See https://theme.co/apex/forum/t/setup-updating-your-themes-and-plugins/62 for more details.
If you’re using a caching plugin, clear all caches then temporarily deactivate your caching plugin and other optimization plugins. Also, clear your browser’s cache.
If you’re using a CDN, please clear the CDN’s cache and temporarily disable optimization services.
Test for a plugin conflict. You can do this by deactivating all third party plugins, and seeing if the problem remains. If it’s fixed, you’ll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
In the WP Admin Menu, go to Settings > Permalinks and just click the Save Changes button.
Contact your host to increase your allocated memory or do it yourself by adding this code in your wp-config.php
define( 'WP_MEMORY_LIMIT', '256M' );
define( 'WP_MAX_MEMORY_LIMIT', '512M' );
Only if nothing helps in the list, give us your site’s WordPress Login URL, Admin Username and Password in a Secure Note.