Make sure your php version is 7.3 and pdo MySQL extension are installed.
Make sure MySQL service is running on your server.
Saurabh Thank you
but, it happens sometime, for 5 minutes or for 10 minutes. without any modification or edit it will be back automatically to normal working condition,
abgaadelivery Then it was your server fault. The MySQL instance might have crashed and been restarted by your hosting provider. (Or your hosing provider made an update or something on your account instance)
