Hey This is clearly the issue of your Database connection.
Please make sure these things.
1. Your Database Host is correct in lib >> db_connect.php ? is it localhost if yes then confirm with your web hosting your correct DB host
2. Please make sure your Database name is correct.
3. Your Database User name and password is correct.
4. Your Database is linked with your user.
Please confirm these things and your issue is core related to your DB connection. thanks