I have a lenovo b470 laptop. due to some proble i have format it but after then my function key not working.

Do they work when you use external keyboard?

In your 'Start' menu click on 'Run' and type in services.msc
You'll see a long list called Services (Local), click on the name parameter to have them sorted alphabetically.
Find the one called VAIO Event Service. Double click this and ensure that for Startup Type it says 'automatic', if it does not, change it to 'automatic' and click 'apply'.
Then be sure below that, that the Service Status says 'Started'.