If WordPress does not run properly anymore, mostly the typical candidates come into question as possible error cause: Outdated core, outdated theme or plugin, problems due to changed PHP version, plugin conflicts with each other or too little memory.
Not all errors can be detected at first glance. So before you invest too much time, you should first read the error log. For this, a corresponding addition must be made in the wp-config.php (depending on the hoster, additional settings may have to be made there).
define( 'WP_DEBUG', true ); define( 'WP_DEBUG_LOG', true ); define( 'WP_DEBUG_DISPLAY', false );
The corresponding log file should then be found on the server under /wp-content/debug.log.