You was on 4.2 with preloader on CIOS rev 15 and then you formatted it using the official format option from the system menu? If this is the case then read below otherwise give full details of what you did exactley to brick it, giving as much detail as possible.
If you formated from within system menu you removed your system menu but still have access to preloader, you need to reinstall your system menu, you will need to run a copy of wad manger from within preloader to install your correct system menu wad, couldnt hurt to reinstall IOS 70 at same time, once that is done you should be able to boot back to the system menu from within preloader.