Ubuntu crontab is not triggering when site is in maintenance mode and also revert to online I am using the Ultimate cron module and I have set the main Drupal cron task in crontab to run every 5 minutes. 

I have put my site in maintenance mode. Drupal cron tasks are not running when site is in maintenance mode, and it is not resuming its operation when I changed the site back to online.

Please suggest.
When the site is in maintenance mode it makes sense that regular cron tasks are halted. For example email notifications about new content should not be sent if you are working with content in maintenance mode - unless you want users to go to a site down page.

If you manually trigger the cron to run after disabling maintenance mode it should restart the jobs.