Occasionally, it may be necessary to start (or restart) your Dynamics NAV Server instance.
This is accomplished through the use of the Microsoft Dynamics NAV Server Administration tool.
Once you have started the Microsoft Dynamics NAV Server Administration tool for your version of NAV...
note: you will typically need to be local admin on the computer
In the left pane, choose a Microsoft Dynamics NAV Server computer.
Unless you are administering a remote computer, this is Microsoft Dynamics NAV (local).
In the center pane, choose the instance name
In the right pane, choose Start (if the Instance is currently "Stopped") or Restart
Reference: MSDN article How to: Start, Stop, Restart, or Remove a Microsoft Dynamics NAV Server Instance