You cannot restore a backup of a later version of iOS (in this case 7) to a device running a lower number of iOS (6 in your case). You should have made a backup (plus a desktop copy in case iTunes copies over it) of the iOS 6 running device before you experimented with the beta! It's really common sense, but it eludes users here for some reason. You can get your info back once you either a). Return to an iOS 7 beta or b). When the official public release is available. When I use betas I make a backup of the official software first in iTunes, plus another copy on the desktop, plus I turn off iCloud backups for the duration of the betas.
Ps. You can restore from backup using iTunes when using betas. It will not lock out your device if the UDID is not registered.
Thank you captain obvious! I don't think he asked anyone to tell him the mistake he made I think he was asking how to fix it. Not everyone has done this more than once. If you have a way to get around it that great if not what is the point of even commenting on here? Just to say how cool you are because you thought to make a backup on the computer? I NEVER even connect my phone to the computer anymore. That's the point of the "over the air" updates, right?