How can I use Drush 8 command to enable the site set up bin the Acquia DevDesktop to online mode? I forgot to put the site back online when I'd finished updating the site. How can I use Drush command to enable the site to online mode?
You are talking about maintenance mode. If you had use the correct term, you would have been pointed to for example the following question which has the info you need:

drush sset system.maintenance_mode 0