Rename your .htaccess file to something like .htaccess_old and then try reloading your site. If this fixes the issue, then you can regenerate the .htaccess file by going to Settings > Permalinks in your WordPress dashboard and clicking the "Save Changes" button.
Deactivate all plugins and try to reload your site. If this fixes the issue, then reactivate each plugin one by one and reload your site until you find the plugin causing the issue.
Add the following code to your wp-config.php file to increase the PHP memory limit:
Make sure your PHP version is up to date and compatible with your WordPress version. You can check your PHP version by using a plugin like "Display PHP Version".
If none of the above steps work, then contact your hosting provider to check if there are any server-side issues causing the error.