What happens with scheduled workflows during a scheduled server shutdown?
A server being down would affect the SharePoint timer service which runs ever five min.(by default) The Nintex Workflow Scheduler job runs with in the SharePoint Timer job.
If the outage persist past the time a workflow was scheduled to kick off, then the workflow wouldn't not kick out due to the Nintex workflow scheduler not running.
Once the server comeback on line and the Nintex Workflow Scheduler job is running, it will work through it's queue of workflow of which will run late.