Try flashing the stock firmware using recovery.
Is it on GB or ICS?
Anyway, download the
stock rom
Put in the root of your sd card.
Enter recovery (To start Recovery (ClockworkMod) from power-off.: Press & hold Vol-Up, then press & hold Power. Shortly after the plain white Samsung logo appears, release both buttons. ClockworkMod should start after the color Samsung/T-Mobile Exhibit II logo.)
Boot into ClockworkMod. Flash file. That's it, phone is stock. Wipe Data/Cache/Dalvik prior to reboot.