Magento2: Service Temporarily Unavailable

Magento 2 maintenance mode file has the name .maintenance.flag (with the leading dot) and is located in the var folder.
But HTTP 503 code response can have different causes. For example, the cause could be a webserver misconfiguration.

• Delete the file called .maintenance.flag located in the var folder.
• If appear Exception printing is disabled by default for security reasons, please check the error log.
• If the error log prompts Please upgrade your database, run:

php bin/magento setup:upgrade

from the Magento root directory.