Wow I have the same problem!!
I just got my new sony vaio f series laptop today in the mail literally an hour and a half ago. All was fine and now it's not working (on a different laptop right now). I can press Fn+F5/F6/F12 but that's it. I can't figure out if I hit some button that turns it off or what.
Hey, vidood again! I got it to work! This is what I did...
-On your computer there should be a button that says "Assist" (like up on the top right-hand side of the keyboard). Click that. It should bring up "Sony Care"
-Then click "Input & Output"
-It might ask "Are you using a wireless keyboard"? say, "No"
-Click "Keyboard problems"
-Follow that until finished
After that my keyboard went back to working! Not sure what had happened in the first place, but all is well now. Hope this works for you!
I would first try using an external USB keyboard. This will verify that the keyboard is in fact defective. If you find that it is defective then ebay is a good source for replacement keyboards. Try to find the maintenance manual to determine the method to remove the current keyboard.