Your website appears to be accessible as normal. However, I suspect that you are referring to wp-admin. An error message appears there. What you see there is an error from the “Auto Featured Image (Auto Post Thumbnail)” plugin that you are using.
My guess is that you have installed a faulty version of this plugin, perhaps it was added to your project through automatic updates, if you use them.
To solve this, you need to delete the plugin. Since you can’t access the backend, you’ll have to do this via FTP. Log in via FTP and delete the directory /wp-content/plugins/auto-post-thumbnail/. Your host’s support team can help you connect via FTP. After that, you should be able to log in again.
Once you are back in, you can of course reinstall the plugin. I suspect their developers have already solved the problem today, as I can see from the changelog: https://plugins.trac.ww.wp.xz.cn/browser/auto-post-thumbnail/trunk/CHANGELOG.md?rev=3462494 – If you have any questions about the plugin, please contact their support forum: https://ww.wp.xz.cn/support/plugin/auto-post-thumbnail/
