503 error is generated from server side. There are many reasons for this error on your website as follow:
If your website is hosted on Windows server, your website’s application pool is not running. There can be different reasons for this and your hosting provider should be able to share more information on it.
If your website is on Linux server, it is possible that your account is hitting its allowed CPU/RAM limit. You should be able to review the usage from your cPanel.
In most cases, whenever your hosting account is reaching its limit, you may notice 503 error in your website. Your hosting provider should be able to share more information on this.