I know this is an old post but for anyone else that gets this it's just a case of re-installing the OS though iTunes. Basically the Jailbreak has gone bad. You might be able to fix it though direct root access and a command line but it's normally not worth it as it would take ages and might not work.
- Basically plug your iPhone into a PC or Mac with iTunes and back it up (if it will let you. If not then lets hope you have a recent backup or you'll be starting again! ;P)
* If it doesn't appear in iTunes then hold the Power and Home button for 10 seconds then release JUST the power button leaving the home button pressed until the PC/Mac ether see's the phone or starts installing drivers (it will need different drivers for DFU mode which is what you have just put it in)
- Once iTunes see's your phone (ether DFU mode or normal) click the "Update" button and follow the instructions. It will download the newest copy of iOS (you can't install old versions so if the newest version isn't Jailbreakable your kinda going to have to wait till the JB it! ). It will likely take a while to download and install and your iPhone will restart a few times. Once it's done you can ether restore a backup already stored on your computer (or iCloud if you where clever enough to turn that one! :P) and then Jailbreak again with the latest FREE (don't ever pay instead donate to the people that really spent the time working out a jb!). Currently I think it's Evasi0n. iOS 7 - 7.0.4 Jailbreak: Download Evasi0n 7.0.6 For iPhone 5s, 5c, 5, iPad, iPod touch [Windows / Mac] | Redmond Pie