You should try to restart the "Apple Mobile Device" service.
Here is a way to do this:
- In the windows start pane type in "Services" and look for the Services icon.
- Within Services, try to find APPLE MOBILE SERVICES,
- Right click on APPLE MOBILE SERVICES and select "restart".
- After a while iTunes should find your iPod nano automatically and the greyed out SYNC option should be back on.