Computer make and model? Operating system used? EXACTLY how are you trying to turn it on?
And because this error is usually related to your Firewall, but could also be affecting Defender, it appears like you may be infected with malware. So... Please click on the link below and follow the steps EXACTLY in the order they're given..
“Expand” the post titled: "Please try this" at the link below:
http://www.cnet.com/forums/post/f742c795-5881-433b-a29b-6d758efe5cd3/
Hope this helps and let us know more.
For some reason it has turned off and when I try and turn it on I receive the error message:
Windows cannot change some of your settings
Error code 0x8007042c
I have run SpyBot and Malwarebytes but neither have spotted any problems. How can I fix this?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic