HTTP Error 503. The service is unavailable.
You will receive this error when browsing website hosted on your server. You see this error if you are hosted on a windows server. If you have administrator details, access the server via RDP and restart IIS. If you face issues when restarting IIS, refer IIS not starting. IIS once restarted, you should be able to access the website.
If you are hosted on a shared server, contact your web hosting & ask them to restart required services.