Cause of the cause:
1. Replace new theme
2. Install or upgrade the plugin
3. Upgrade WordPress version
The root of the problem is the incompatibility between themes, plug-ins, and WordPress itself:
1. Some topics integrate a number of powerful features that may conflict with Plug-ins now in use
2. Some plugin versions or themes are incompatible with the WordPress version you are using
Solution:
Method One: Rename the currently enabled topic through FTP
Renaming the currently installed Theme folder will force WordPress to automatically select the default theme and then load it properly.
1. Find the Wp-contents/themes folder through FTP, and rename the currently installed theme folder. For example: Name the Twentytwenty as Twentytwenty-temp
2. Login WordPress Background, disable all plug-ins, and then FTP will be the original theme name back, reactivated,
1 if all is normal, the description is this topic and a plug-in conflict, activate Plug-ins, to find the conflict of plug-ins and disable;
2 If there is a problem with the reactivation of the theme, this topic is incompatible with the version of WordPress you are currently using, or there is an error in the theme.
Method Two: Reset the plugin folder through FTP
1. Login via FTP, rename the Plugins folder under Wp-contents to Plugins-temp
2. Create a new folder in the same directory for plugins, login back to WordPress background,
1 If the problem persists, the description is that the theme is not compatible with your WordPress, or the theme of the error;
2 If everything is normal, the description is a plug-in with the current version of WordPress or theme conflicts, the plug-in from Plugins-temp to plugins, and then reactivate, find incompatible plug-ins and disable.