The license manager is run as a service (ESRI License Manager). It should start automatically when you reboot your computer.
Go to Start > Programs > ArcGIS > License Manager > License Manager Tools
Choose the Service/License File tab.
Select 'Configuration using Services'. Click on the 'ESRI License Manager' service to select it.
Click on the Start/Stop/Reread tab and press Start Server.
To stop the service, press Stop Server.
Windows NT:
Go to Control Panel > Services
Click on ESRI License Manager.
Click Stop or Start.
Windows 2000:
Go to Control Panel > Administrator Tools > Services
Click on ESRI License Manager.
Click Stop or Start.