Mysql went down during Magento 2 installation on localhost and put site to maintenance mode without finalising installation.
Quick 3 steps solutions:
var/.maintenance.flag
filephp bin/magento setup:upgrade
php bin/magento admin:user:create --admin-user='adminuser' --admin-password='adminpassword' --admin-firstname='Name' --admin-lastname='Surname' --admin-email='admin@email.com'