You can try putting the iPhone into DFU mode and see if that will let you restore and remove the jailbreak:
http://www.iclarified.com/entry/index.php?enid=1034
If that works, you will not be able to restore iOS 6; you'll only be able to get the most current release, which as of right now is iOS 7.0.4.
Some jailbreaks cannot be removed and in some cases will permanently disable your iPhone. If that happens to you, then you will have to buy a new iPhone; Apple will provide no support, warranty or out-of-warranty, for a jailbroken device.
You're on your own from here. We can provide no further help with a jailbroken iPhone in these forums.
Good luck.