In the Overview page with a list of all the monitors, there is a drop down section towards to the top of the screen labeled "Active". Here you can adjust which monitors can be brought to view.
Monitors are always in one of two different states: Active and Paused. Users have the ability to view both with the All selection.
Many users run into the issue where they pause a monitor and can't find it when they log back into NUVI.
Note: For users with multiple companies, make sure you are in the correct company if you still can't find the monitor.