I have been using my L750 for about 3 weeks now and suddenly the function keys (using the FN + F keys) have ceased to work.
Go to All Programs -> Flash Cards Utility and restart the Flash Cards.
The point is that this Utility controls the FN buttons and this should be the first step in order to get the button working again.
If this does not help, reinstall the VAP (value added package)
Regarding the power off issue:
Click right on the Start button -> Properties
Now choose Start Menu tab
Here in the point power button action choose Shut down