iOS 7 Betas Expire
Many developers are reporting that their devices are not working. This is because the betas have expired. Developers that have not updated to the public version of iOS 7 are now locked out of their devices. Luckily, there is a fix.
If you have Find My iPhone off you will need to download iTunes 11.1. Then download iOS 7.0.2 on your computer. Plug in your device and open iTunes. On the device page hold option on a Mac or shift on a PC while clicking on “Restore iPhone”. A page will come up and you will then select the iOS 7.0.2 file you downloaded.
If you have Find My iPhone on you will need to put your device in DFU restore mode. You will get two options, update or restore. From there you will do the same thing. There is full information on how to do this here.